题题题题题解

#include <bits/stdc++.h>
using namespace std;
int main(){
    int n;
    cin >> n;
    int two = 0,three = 1;
    for(int i = 1;i < n;i++){
        int th = three;
        three = two + three;
        two = two + th * 2;
    }
    cout << two * 2 + three * 3;
    return 0;
}