数学·概率

#include <algorithm>
#include <cstdio>
#include <cstring>
#include <string>
#include <queue>
#include <map>
#include <set>
#include <ctime>
#include <cmath>
#include <iostream>
#include <assert.h>
#define PI acos(-1.)
#pragma comment(linker, "/STACK:102400000,102400000")
#define max(a, b) ((a) > (b) ? (a) : (b))
#define min(a, b) ((a) < (b) ? (a) : (b))
#define mp make_pair
#define st first
#define nd second
#define keyn (root->ch[1]->ch[0])
#define lson (u << 1)
#define rson (u << 1 | 1)
#define pii pair<int, int>
#define pll pair<ll, ll>
#define pb push_back
#define type(x) __typeof(x.begin())
#define foreach(i, j) for(type(j)i = j.begin(); i != j.end(); i++)
#define FOR(i, s, t) for(int i = (s); i <= (t); i++)
#define ROF(i, t, s) for(int i = (t); i >= (s); i--)
#define dbg(x) cout << x << endl
#define dbg2(x, y) cout << x << " " << y << endl
#define clr(x, i) memset(x, (i), sizeof(x))
#define maximize(x, y) x = max((x), (y))
#define minimize(x, y) x = min((x), (y))
//using namespace std;
typedef long long ll;
const int int_inf = 0x3f3f3f3f;
const ll ll_inf = 0x3f3f3f3f3f3f3f3f;
const int INT_INF = (int)((1ll << 31) - 1);
const double double_inf = 1e30;
const double eps = 1e-14;
typedef unsigned long long ul;
inline int readint(){
int x;
scanf("%d", &x);
return x;
}
inline int readstr(char *s){
scanf("%s", s);
return strlen(s);
}
//Here goes 2d geometry templates
struct Point{
double x, y;
Point(double x = 0, double y = 0) : x(x), y(y) {}
};
typedef Point Vector;
Vector operator + (Vector A, Vector B){
return Vector(A.x + B.x, A.y + B.y);
}
Vector operator - (Point A, Point B){
return Vector(A.x - B.x, A.y - B.y);
}
Vector operator * (Vector A, double p){
return Vector(A.x * p, A.y * p);
}
Vector operator / (Vector A, double p){
return Vector(A.x / p, A.y / p);
}
bool operator < (const Point& a, const Point& b){
return a.x < b.x || (a.x == b.x && a.y < b.y);
}
int dcmp(double x){
if(abs(x) < eps) return 0;
return x < 0 ? -1 : 1;
}
bool operator == (const Point& a, const Point& b){
return dcmp(a.x - b.x) == 0 && dcmp(a.y - b.y) == 0;
}
double Dot(Vector A, Vector B){
return A.x * B.x + A.y * B.y;
}
double Len(Vector A){
return sqrt(Dot(A, A));
}
double Angle(Vector A, Vector B){
return acos(Dot(A, B) / Len(A) / Len(B));
}
double Cross(Vector A, Vector B){
return A.x * B.y - A.y * B.x;
}
double Area2(Point A, Point B, Point C){
return Cross(B - A, C - A);
}
Vector Rotate(Vector A, double rad){
//rotate counterclockwise
return Vector(A.x * cos(rad) - A.y * sin(rad), A.x * sin(rad) + A.y * cos(rad));
}
Vector Normal(Vector A){
double L = Len(A);
return Vector(-A.y / L, A.x / L);
}
void Normallize(Vector &A){
double L = Len(A);
A.x /= L, A.y /= L;
}
Point GetLineIntersection(Point P, Vector v, Point Q, Vector w){
Vector u = P - Q;
double t = Cross(w, u) / Cross(v, w);
return P + v * t;
}
double DistanceToLine(Point P, Point A, Point B){
Vector v1 = B - A, v2 = P - A;
return abs(Cross(v1, v2)) / Len(v1);
}
double DistanceToSegment(Point P, Point A, Point B){
if(A == B) return Len(P - A);
Vector v1 = B - A, v2 = P - A, v3 = P - B;
if(dcmp(Dot(v1, v2)) < 0) return Len(v2);
else if(dcmp(Dot(v1, v3)) > 0) return Len(v3);
else return abs(Cross(v1, v2)) / Len(v1);
}
Point GetLineProjection(Point P, Point A, Point B){
Vector v = B - A;
return A + v * (Dot(v, P - A) / Dot(v, v));
}
bool SegmentProperIntersection(Point a1, Point a2, Point b1, Point b2){
//Line1:(a1, a2) Line2:(b1,b2)
double c1 = Cross(a2 - a1, b1 - a1), c2 = Cross(a2 - a1, b2 - a1),
c3 = Cross(b2 - b1, a1 - b1), c4 = Cross(b2 - b1, a2 - b1);
return dcmp(c1) * dcmp(c2) < 0 && dcmp(c3) * dcmp(c4) < 0;
}
bool OnSegment(Point p, Point a1, Point a2){
return dcmp(Cross(a1 - p, a2 - p)) == 0 && dcmp(Dot(a1 - p, a2 -p)) < 0;
}
Vector GetBisector(Vector v, Vector w){
Normallize(v), Normallize(w);
return Vector((v.x + w.x) / 2, (v.y + w.y) / 2);
}

bool OnLine(Point p, Point a1, Point a2){
Vector v1 = p - a1, v2 = a2 - a1;
double tem = Cross(v1, v2);
return dcmp(tem) == 0;
}
struct Line{
Point p;
Vector v;
Point point(double t){
return Point(p.x + t * v.x, p.y + t * v.y);
}
Line(Point p, Vector v) : p(p), v(v) {}
};
struct Circle{
Point c;
double r;
Circle(Point c, double r) : c(c), r(r) {}
Circle(int x, int y, int _r){
c = Point(x, y);
r = _r;
}
Point point(double a){
return Point(c.x + cos(a) * r, c.y + sin(a) * r);
}
};
int GetLineCircleIntersection(Line L, Circle C, double &t1, double& t2, std :: vector<Point>& sol){
double a = L.v.x, b = L.p.x - C.c.x, c = L.v.y, d = L.p.y - C.c.y;
double e = a * a + c * c, f = 2 * (a * b + c * d), g = b * b + d * d - C.r * C.r;
double delta = f * f - 4 * e * g;
if(dcmp(delta) < 0) return 0;
if(dcmp(delta) == 0){
t1 = t2 = -f / (2 * e); sol.pb(L.point(t1));
return 1;
}
t1 = (-f - sqrt(delta)) / (2 * e); sol.pb(L.point(t1));
t2 = (-f + sqrt(delta)) / (2 * e); sol.pb(L.point(t2));
return 2;
}
double angle(Vector v){
return atan2(v.y, v.x);
//(-pi, pi]
}
int GetCircleCircleIntersection(Circle C1, Circle C2, std :: vector<Point>& sol){
double d = Len(C1.c - C2.c);
if(dcmp(d) == 0){
if(dcmp(C1.r - C2.r) == 0) return -1; //two circle duplicates
return 0; //two circles share identical center
}
if(dcmp(C1.r + C2.r - d) < 0) return 0; //too close
if(dcmp(abs(C1.r - C2.r) - d) > 0) return 0; //too far away
double a = angle(C2.c - C1.c); // angle of vector(C1, C2)
double da = acos((C1.r * C1.r + d * d - C2.r * C2.r) / (2 * C1.r * d));
Point p1 = C1.point(a - da), p2 = C1.point(a + da);
sol.pb(p1);
if(p1 == p2) return 1;
sol.pb(p2);
return 2;
}
int GetPointCircleTangents(Point p, Circle C, Vector* v){
Vector u = C.c - p;
double dist = Len(u);
if(dist < C.r) return 0;//p is inside the circle, no tangents
else if(dcmp(dist - C.r) == 0){
// p is on the circles, one tangent only
v[0] = Rotate(u, PI / 2);
return 1;
}else{
double ang = asin(C.r / dist);
v[0] = Rotate(u, -ang);
v[1] = Rotate(u, +ang);
return 2;
}
}
int GetCircleCircleTangents(Circle A, Circle B, Point* a, Point* b){
//a[i] store point of tangency on Circle A of tangent i
//b[i] store point of tangency on Circle B of tangent i
//six conditions is in consideration
int cnt = 0;
if(A.r < B.r) { std :: swap(A, B); std :: swap(a, b); }
int d2 = (A.c.x - B.c.x) * (A.c.x - B.c.x) + (A.c.y - B.c.y) * (A.c.y - B.c.y);
int rdiff = A.r - B.r;
int rsum = A.r + B.r;
if(d2 < rdiff * rdiff) return 0; // one circle is inside the other
double base = atan2(B.c.y - A.c.y, B.c.x - A.c.x);
if(d2 == 0 && A.r == B.r) return -1; // two circle duplicates
if(d2 == rdiff * rdiff){ // internal tangency
a[cnt] = A.point(base); b[cnt] = B.point(base); cnt++;
return 1;
}
double ang = acos((A.r - B.r) / sqrt(d2));
a[cnt] = A.point(base + ang); b[cnt++] = B.point(base + ang);
a[cnt] = A.point(base - ang); b[cnt++] = B.point(base - ang);
if(d2 == rsum * rsum){
//one internal tangent
a[cnt] = A.point(base);
b[cnt++] = B.point(base + PI);
}else if(d2 > rsum * rsum){
//two internal tangents
double ang = acos((A.r + B.r) / sqrt(d2));
a[cnt] = A.point(base + ang); b[cnt++] = B.point(base + ang + PI);
a[cnt] = A.point(base - ang); b[cnt++] = B.point(base - ang + PI);
}
return cnt;
}
Point ReadPoint(){
double x, y;
scanf("%lf%lf", &x, &y);
return Point(x, y);
}
Circle ReadCircle(){
double x, y, r;
scanf("%lf%lf%lf", &x, &y, &r);
return Circle(x, y, r);
}
//Here goes 3d geometry templates
struct Point3{
double x, y, z;
Point3(double x = 0, double y = 0, double z = 0) : x(x), y(y), z(z) {}
};
typedef Point3 Vector3;
Vector3 operator + (Vector3 A, Vector3 B){
return Vector3(A.x + B.x, A.y + B.y, A.z + B.z);
}
Vector3 operator - (Vector3 A, Vector3 B){
return Vector3(A.x - B.x, A.y - B.y, A.z - B.z);
}
Vector3 operator * (Vector3 A, double p){
return Vector3(A.x * p, A.y * p, A.z * p);
}
Vector3 operator / (Vector3 A, double p){
return Vector3(A.x / p, A.y / p, A.z / p);
}
double Dot3(Vector3 A, Vector3 B){
return A.x * B.x + A.y * B.y + A.z * B.z;
}
double Len3(Vector3 A){
return sqrt(Dot3(A, A));
}
double Angle3(Vector3 A, Vector3 B){
return acos(Dot3(A, B) / Len3(A) / Len3(B));
}
double DistanceToPlane(const Point3& p, const Point3 &p0, const Vector3& n){
return abs(Dot3(p - p0, n));
}
Point3 GetPlaneProjection(const Point3 &p, const Point3 &p0, const Vector3 &n){
return p - n * Dot3(p - p0, n);
}
Point3 GetLinePlaneIntersection(Point3 p1, Point3 p2, Point3 p0, Vector3 n){
Vector3 v = p2 - p1;
double t = (Dot3(n, p0 - p1) / Dot3(n, p2 - p1));
return p1 + v * t;//if t in range [0, 1], intersection on segment
}
Vector3 Cross(Vector3 A, Vector3 B){
return Vector3(A.y * B.z - A.z * B.y, A.z * B.x - A.x * B.z, A.x * B.y - A.y * B.x);
}
double Area3(Point3 A, Point3 B, Point3 C){
return Len3(Cross(B - A, C - A));
}
class cmpt{
public:
bool operator () (const int &x, const int &y) const{
return x > y;
}
};

int Rand(int x, int o){
//if o set, return [1, x], else return [0, x - 1]
if(!x) return 0;
int tem = (int)((double)rand() / RAND_MAX * x) % x;
return o ? tem + 1 : tem;
}
////////////////////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////////////////////
void data_gen(){
srand(time(0));
freopen("in.txt", "w", stdout);
int times = 100;
printf("%d\n", times);
while(times--){
int r = Rand(1000, 1), a = Rand(1000, 1), c = Rand(1000, 1);
int b = Rand(r, 1), d = Rand(r, 1);
int m = Rand(100, 1), n = Rand(m, 1);
printf("%d %d %d %d %d %d %d\n", n, m, a, b, c, d, r);
}
}

struct cmpx{
bool operator () (int x, int y) { return x > y; }
};
int debug = 1;
int dx[] = {0, 0, 1, 1};
int dy[] = {1, 0, 0, 1};
//-------------------------------------------------------------------------
double mem[200000];
bool vis[200000];
double solve(int n, int k){
if(n == 1) return 0;
if(k >= n - 1) return (double)n;
int p = min(n - 1, 2 * k);
double ans = n;
double pre = 1.;
int sigma_0 = n - 1 - min(k, n - 1);
if(sigma_0 < p) return n;
FOR(i, 0, p - 1) pre *= (double)(sigma_0 - i) / (n - 1 - i);
clr(vis, 0);
mem[sigma_0] = pre;
vis[sigma_0] = 1;
FOR(i, 0, n - 1){
int sigma_i = n - 1 - min(k, i) - min(k, n - 1 - i);
if(sigma_i < p) continue;
if(vis[sigma_i]){
ans -= mem[sigma_i];
continue;
}
vis[sigma_i] = 1;
ans -= (mem[sigma_i] = mem[sigma_i + 1] / (double)(1 + sigma_i) * (double)(sigma_i - p + 1));
}
return ans;
}
//-------------------------------------------------------------------------
int main(){
//data_gen(); return 0;
//C(); return 0;
debug = 0;
///////////////////////////////////////////////////////////////////////////////////////////////////////////////
if(debug) freopen("in.txt", "r", stdin);
//freopen("in.txt", "w", stdout);
int n, k;
int kase = 0;
while(~scanf("%d%d", &n, &k) && n){
double ans = solve(n, k);
printf("Case %d: %.4f\n", ++kase, ans);
}
//////////////////////////////////////////////////////////////////////////////////////////////////////////////
return 0;
}