51nod 1013 3的幂的和

#include <bits/stdc++.h>using namespace std;long long mod=1e9+7;long long powmod(long long x,long long p){long long ret=1;while(p){if(p&1)ret=ret*x%mod;x=x*x%mod;p>>=1;} return ret;}int main(){long long n;while(cin>>n){printf("%lld\n",(powmod(3,n+1)+mod-1)*powmod(2,mod-2)%mod);}}