UVa Q580

//UVa Q580
#include <bits/stdc++.h>
using namespace std;

int32_t main(){
    int arr[31] = {1,2,4};
    for(int i = 3; i <= 30; ++i)arr[i] = arr[i-1] + arr[i-2] + arr[i-3];
    int n;
    while(cin >> n){
        if(n == 0)break;
        cout << (1<<n) - arr[n] << '\n';
    }
    return 0;
}