#include<iostream>
using namespace std;
typedef long long ll;

int main(){
    int n;
    cin >> n;

    ll dp[100][2] = {};
    dp[0][0] = 1;

    for(int i = 0; i < n; i++){
        dp[i+2][1] += dp[i][0];
        dp[i+1][0] += dp[i][0]+dp[i][1];
    }

    cout << dp[n][0]+dp[n][1] << endl;

    return 0;
}