#include <bits/stdc++.h>
using namespace std;
using ll = long long;
constexpr ll inf = (1LL << 62);
template <typename T>
bool chmax(T& a, const T& b)
{
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
template <typename T>
bool chmin(T& a, const T& b)
{
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
ll dp[60];
int main()
{
    ll n;
    cin >> n;
    dp[0] = 1;
    dp[1] = 2;
    for (ll i = 2; i < n; ++i) {
        dp[i] = dp[i - 1] + dp[i - 2];
    }
    cout << dp[n - 1] << endl;
}