文件目录

#include <iostream>
using namespace std;
int main()
{
    int f[25] = {0, 3, 7, 17};
    for (int i = 4; i <= 20; i++)
    {
        f[i] = f[i - 1] * 2 + f[i - 2];
    }
    int n;
    cin >> n;
    cout << f[n];
    return 0;
}