#include<bits/stdc++.h>
using namespace std;
int n;
int t(int a){
    if(a<=2) return 1;
	return t(a-1)+t(a-2);
}
int main(){
    int s=0;
    cin>>n;
    for(int i=1;i<=n;i++){	
    s=s+t(i);
    } 
   cout<<s;
    return 0;
}
 
 
/**************************************************************
	Problem: 1238
	User: panyuchen
	Language: C++
	Result: Wrong Answer
****************************************************************/