#include <iostream>
#include <sstream>
#include <vector>
#include <algorithm>
#include <string>
#include <functional>
#include <numeric>
#include <ctype.h>
#include <cstdlib>
#include <string.h>
#include <cmath>
#include <cstdio>
#include <map>
#include <queue>
#include <utility>

using namespace std;

const int INF = 1000000007;
const double PI = acos(-1.0);
typedef long long ll;

int main()
{
	int N;
	cin >> N;
	ll dp[51];
	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;
	
}