#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 1e6 + 10, INF = 0x3f3f3f3f, MOD = 1E9 + 7;
ll n, f[55] = {0, 1, 2, 4};

int main(int argc, char* argv[]) {
    cin >> n;
    for (int i = 4; i <= n; i++)
        f[i] = f[i - 1] + f[i - 2] + f[i - 3];
    cout << f[n];
    return 0;
}