#include <bits/stdc++.h>
#define ll long long
using namespace std;
 
ll n,ans,v[51] = {0,1,2};

int main()
{
    cin >> n;

    for(int i = 3; i <= n; i++) v[i] = v[i-1] + v[i-2];

    cout << v[n] << endl;
    return 0;
}