using System;
using System.Linq;
using System.Collections.Generic;

public class yukicoder
{
    public static void Main()
    {
		long n = long.Parse(Console.ReadLine());
		long ans = 1;
		
		for(long i = 3; i < n; i = i + 2)
		{
			ans = ans + i;
		}
		
		Console.WriteLine(ans);

    }
}