#include<iostream>
#define ll long long

using namespace std;

ll dp[55];

int main(){
    ll n;
    cin >> n;
    dp[0] = 0;
    dp[1] = 1;
    dp[2] = 2;
    for(int i=3;i <= n;i++){
        dp[i] = dp[i-1] + dp[i-2];
    }
    cout << dp[n] << endl;
    return 0;
}