Problem:
Score : 100 points

Problem Statement

Find the number of ways to choose a pair of an even number and an odd number from the positive integers between 1 and K (inclusive). The order does not matter.

Constraints

2\leq K\leq 100

K is an integer.

Input

Input is given from Standard Input in the following format:

K

Output

Print the number of ways to choose a pair of an even number and an odd number from the positive integers between 1 and K (inclusive).

Sample Input 1

3

Sample Output 1

2

Two pairs can be chosen: (2,1) and (2,3).

Sample Input 2

6

Sample Output 2

9

Sample Input 3

11

Sample Output 3

30

Sample Input 4

50

Sample Output 4

625

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;


class Program
{
    static void Main(string[] args)
    {
        int k = int.Parse(Console.ReadLine());
        int ans=0;
        if (k % 2 == 0)
        {
            ans = (int)Math.Pow((k/2),2.0d);
        }
        else
        {
            ans = ((int)Math.Pow(k,2)-1)/4;
        }



        Console.WriteLine(ans); 
        
    }
}