51Nod 1013 3的幂的和（快速幂+逆元）

#include <iostream>
#include <algorithm>
#include <string>

#define MOD 1000000007
using namespace std;

long long quick_pow(long long a, long long b, long long c){
    long long ans = 1;
    a = a%c;
    while (b > 0){
        if (b % 2 == 1){
            ans = (ans*a) % c;
        }
        b = b / 2;
        a = (a*a) % c;
    }
    return ans;
}

int main(){
    ios::sync_with_stdio(false);
    /*
    freopen("in.txt", "r", stdin);
    freopen("out.txt", "w", stdout);
    */
    long long x;
    while (cin >> x){
        x++;
        long long ans = quick_pow(3, x, MOD);
        ans--;
        ans = ans * 500000004 % MOD;
        cout << ans << endl;
    }
    //fclose(stdin);
    //fclose(stdout);
    //system("pause");
    return 0;
}