求大佬

#include<bits/stdc++.h>
using namespace std;
unsigned long long a,b,p,sum,j[10086],cnt;
int main(){
	cin>>a>>b>>p;
	while(a!=0){
		j[++cnt]=a%2;
		a/=2;
	}
	for(int i=1;i<=cnt;i++){
		if(j[i]){
			sum=sum%p+int (pow(2,i-1))%p *b%p;
		}
	}
	cout<<sum%p;
}