HDU 4990 Reading comprehension

#include<cstdio>
#include<cstring>
#include<cmath>
#include<iostream>
#include<algorithm>
using namespace std;

long long n,MOD;

long long cal(long long a,long long b,long long mod)
{
long long c=1;
while(b!=0)
{
if(b%2==1) c=(c*a)%mod,b--;
else a=(a*a)%mod,b=b/2;
}
return c;
}

int main()
{
while(cin>>n>>MOD)
{
if(n%2==1)
cout<<(cal(2,n+1,MOD*3)-1)/3<<endl;
else
cout<<(cal(2,n+1,MOD*3)-2)/3<<endl;
}
return 0;
}