Codeforce Round #438 C.Bus（贪心）

#include <iostream>
#include <cstdio>
#include <fstream>
#include <algorithm>
#include <cmath>
#include <deque>
#include <vector>
#include <queue>
#include <string>
#include <cstring>
#include <map>
#include <stack>
#include <set>
#define Max(a,b) a>b?a:b
#define Min(a,b) a>b?b:a
#define mem(a,b) memset(a,b,sizeof(a))
using namespace std;
typedef long long ll;
int dir[4][2]= {{1,0},{-1,0},{0,1},{0,-1}};
const double eps = 1e-6;
const double Pi = acos(-1.0);
const int INF=0x3f3f3f3f;
ll fuel[10010];
ll a,b,f,k;
ll dis,ans;
ll ind,now;
bool flag;
int main(){
while(~scanf("%lld%lld%lld%lld",&a,&b,&f,&k)){
mem(fuel,0);
flag = true;
fuel[0] = 0;
for(int i = 1; i <= k; i++){
if(i&1) fuel[i] = a*i - (a-f);
else fuel[i] = a*i - f;
}

fuel[k+1] = a*k;
dis = 0,ans = 0,now = 0;
while(dis < a*k){
ind = lower_bound(fuel,fuel+k+2,dis+b) - fuel;
if(fuel[ind] != dis+b){
ind = ind - 1;
}
if(ind == now){
flag = false; break;
}
now = ind;
dis = fuel[now];
ans++;
}
if(flag) printf("%lld\n",ans-1);
else printf("-1\n");
}
return 0;
}