LightOJ 1213 Fantasy of a Summation

#include <iostream>
#include <algorithm>
using namespace std;
typedef long long ll;

ll pow(int a, int b, int m)
{
ll res = 1;
while(b)
{
if(b&1) res = res*a%m;
a = a*a%m;
b >>= 1;
}
return res;
}

int main()
{
int t,time = 0;
ll n,k,mod;
ll sum,num,n2;
cin >> t;
while(t--)
{
sum = 0;
cin >> n >> k >> mod;
for(int i = 0; i < n; ++i)
{
cin >> num;
sum = (sum+num)%mod;
}
n2 = pow(n,k-1,mod);
n2 = n2*k%mod;
cout << "Case " << ++time << ": " << sum*n2%mod <<endl;
}
return 0;
}