快速乘

#include<bits/stdc++.h>

using namespace std;

#define LL long long

int main(){
    LL a,b,m;
    cin >> a >> b >> m;
    LL ans = 0;
    while(b){
        if(b & 1) ans = (ans + a) % m;
        a = (a<<1) % m;
        b >>= 1;
    }
    cout << ans << endl;
    return 0;
}