#include <iostream>
#include <algorithm>
#include <cmath>
#include <vector>

using namespace std;

int N;
long long int dp[50];

int main()
{
    cin >> N;
    dp[0] = 1;
    dp[1] = 2;

    for(int i = 1; i < N; i++)
    {
        dp[i + 1] = dp[i] + dp[i - 1];
    }

    cout << dp[N - 1] << endl;

    return 0;
}