#include <bits/stdc++.h>
#define int long long
using namespace std;
int a[5005];
signed main(){
	int n;
	cin>>n;
	a[1]=1;a[2]=2;
	for(int i=3;i<=n;i++)
	a[i]=a[i-1]+a[i-2];
	cout<<a[n]<<endl;
	return 0;
}