#pragma GCC optimize("O3", "unroll-loops")

#include <bits/stdc++.h>

using namespace std;

const int inf = 1 << 30;
const long long MOD = 1e9 + 7;

typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef pair<int, int> ii;
typedef vector<ii> vii;
typedef vector<vi> vvi;
typedef vector<bool> vb;

#define FAST ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
#define all(a) a.begin(), a.end()
#define pb push_back
#define files(in, out) freopen(in, "r", stdin); freopen(out, "w", stdout)
#define ppb pop_back
#define ppf pop_front
#define fs first
#define sd second
#define eb emplace_back

template<class T> inline void sort(T &a) { sort(all(a)); }
template<class T> inline T sorted(T a) { sort(a); return a; }
template<class T> inline istream& operator>>(istream& str, vector<T> &a) { for (auto &i : a) str >> i; return str; }

void solve() {
    int n;
    cin >> n;
    vector<ll> dp(n);
    if (n == 1) {
        cout << 1;
        return;
    } else if (n == 2) {
        cout << 2;
        return;
    } else if (n == 3) {
        cout << 4;
        return;
    }
    dp[n - 1] = 1;
    dp[n - 2] = 2;
    dp[n - 3] = 4;
    for (int i = n - 4; i >= 0; --i) {
        dp[i] = dp[i + 1] + dp[i + 2] + dp[i + 3];
    }
    cout << dp[0];
}

signed main() {
    FAST;
    // files("input.in", "output.out");
    int q = 1;
    // cin >> q;
    while (q--) {
        solve();
    }
    return 0;
}