借助快速幂的思想，由平方转化为乘积

#include <iostream>

using namespace std;

int main()
{
    long long a, b, p;
    long long res = 0;

    cin >> a >> b >> p;

    while(b)
    {
        if(b & 1) res = (res + a) % p;
        a = a * 2 % p;
        b >>= 1;
    }

    cout << res % p << endl;

    return 0;
}