#include <stdio.h>
#include <iostream>
using namespace std;
int main()
{
	int n = 0;
	cin >> n;
	double  s = 0;
	int a = 1;
	int b = 1;
	for (int i = 1; i <= n; i++)
	{
		if (i == 1 || i == 2)s = 1;
		if (i > 2)
		{
			s = a + b;
			if (i % 2 != 0)a = a + b;
			else b = a + b;
		}
	}

	printf("%.2lf", s);
	return 0;
}