#include <bits/stdc++.h>
using namespace std;
 
typedef long long ll;
typedef pair<int,int> P;
 
const int INF = 1e9;
const int mod = 1e9+7;
const double EPS = 1e-10;
const double PI = acos(-1.0);

ll dp[60];

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