#include "bits/stdc++.h"
using namespace std;

int main() {
    long int n;
    long long dp[60];
    cin >> n;
    dp[0] = 1;
    dp[1] = 2;
    for (int i = 2; i < n; ++i) {
        dp[i] = dp[i-1] + dp[i-2];
    }

    cout<<dp[n-1]<<endl;
    return 0;
}