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;

namespace Atcoder
{
    public class Program
    {
        public static void Main(string[] args)
        {
            int b = int.Parse(Console.ReadLine());
            if (b % 2 == 0)
            {
                Console.WriteLine(b*b/4);
            }
            else
            {
                Console.WriteLine((b - 1) * (b + 1) / 4);
            }
        }
    }
}