#include<bits/stdc++.h>
using namespace std;
int a[5001];
int louti(int n){
	if(a[n] != 0) 
	return a[n];
	else if(n == 1)
	return 1;
	else if(n == 2)
	return 2;
	else
	return a[n] = louti(n-1)+louti(n-2);//记忆化递归 
}
int main(){
	int n, count = 0, i, j;
	cin >> n;
	count = louti(n);
	cout << count;
	return 0;
}