90. 64位整数乘法

#include <iostream>
using namespace std;

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