#include <iostream>
using namespace std;

int main() {
	
	int n, a=1, b=1,c, suma=0;
	cin >> n;
	
	if (n==1) suma=1;
	if (n==2) suma=2;
	
	if (n>=3)
	{
		suma=2;
		for (int i=3;i<=n;i++)
		{
			c=a+b;
			suma+=c;	//suma=suma+c;
			a=b;
			b=c;
		}
	}
	
	cout << suma << endl;
	
	return 0;
}