using System;

namespace No111_Ababababa
{
    internal class Program
    {
        static void Main(string[] args) {
            int l = int.Parse(Console.ReadLine());
            long a = (l + 1) / 2;
            long b = l - a;
            //long aTotal = (1 + a - 1) * (a - 1) / 2;
            //long bTotal = (1 + b - 1) * (b - 1) / 2;
            //Console.WriteLine(aTotal + bTotal);
            Console.WriteLine((a * (a - 1) + b * (b - 1)) / 2);
        }
    }
}