求大佬看看蒟蒻的快速幂哪里错了？？？

#include <iostream>
using namespace std;
int main() {
    long long a,b,k,s,i,j;
    cin>>a>>b>>k;
    i=a;
    j=b;
    while (b) {
        if (b%2==1)
            s*=a;
        a=a*a;
        b/=2;
        s%=k;
    }
    cout<<i<<'^'<<j<<" mod "<<k<<"="<<s<<endl;
    return 0;
}