POJ 2409-Let it Bead(Polya计数)

#include <stdio.h>
#include <math.h>
#include <string.h>
#include <stdlib.h>
#include <iostream>
#include <sstream>
#include <algorithm>
#include <set>
#include <queue>
#include <stack>
#include <map>
//#pragma comment(linker, "/STACK:102400000,102400000")
using namespace std;
typedef long long LL;
const int inf=0x3f3f3f3f;
const double pi= acos(-1.0);
const double esp=1e-6;
LL gcd(LL a,LL b) {
while(b!=0) {
LL r=b;
b=a%b;
a=r;
}
return a;
}
LL modxp(LL a,LL b) {
LL res=1;
while(b!=0) {
if(b&1) res*=a;
a=a*a;
b>>=1;
}
return res;
}
int main() {
LL c,s,i;
LL ans;
while(~scanf("%lld %lld",&c,&s)) {
if(!c&&!s) break;
ans=0;
for(i=1; i<=s; i++)
ans+=modxp(c,gcd(s,i));
if(s&1) {
ans+=modxp(c,s/2+1)*s;
} else {
ans+=modxp(c,s/2+1)*(s/2);
ans+=modxp(c,s/2)*(s/2);
}
printf("%lld\n",ans/(s*2));
}
return 0;
}