#include <iostream>

using namespace std;
long long dp[60];
int main(){
    int i,n;
    cin >> n;
    dp[1] = 1; dp[2] = 2;
    for(i=3;i<=n;i++){
        dp[i] = dp[i-1] + dp[i-2];
    }
    cout << dp[n] << endl;
}