#include <iostream>
#include <string>
#include <vector>

using namespace std;

#define INF 		2000000007
#define MOD			1000003

#define REP(i,n)	for(int i=0;i<(n);++i)

typedef unsigned long long int ull;

int main(){
	int N;cin >> N;

	/*vector<int> K(N);
	REP(i,N) cin>>K[i];//*/

	vector<ull> DP(N+1);
	int ans;
	
	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;
	return 0;
}