#include<cstdio>
#include<algorithm>
#include<cmath>
#include<iostream>
using namespace std;
long long dp[55]={1};
int  N;
int main(void){
	cin>>N;
	if(3<=N){cout<<N<<endl;}
	else{
		for(int i=4;i<=N;i++){
			dp[i]=dp[i-1]+dp[i-2];
		}
		cout<<dp[N]+3<<endl;
	}
	return 0;
}