#include <bits/stdc++.h>
#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <string>
#include <sstream>
#include <cmath>
#include <stack>
#include <queue>
#include <cctype>
#include <stdio.h>
#include <map>
#include <string.h>
#include <utility>

typedef long long ll;
using namespace std;

typedef pair<int, int> P;

long  dp[51];



int main() {

	dp[0]=0; dp[1] =1; dp[2] =2;

	int n=0;

	cin>>n;
	for(int i=3; i<=n; i++){
		dp[i] = dp[i-1]+dp[i-2];
	}

	cout<<dp[n]<<endl;



	return 0;
}