#include <iostream>
#include <algorithm>
#include <vector>
#include <cstdio>
using namespace std;
typedef long long ll;
const int MAX_N = 51;

ll dp[MAX_N];

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