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 ABC108_A_Pair
{
	internal class Program
	{
		public static void Main(string[] args)
		{
			int k = int.Parse(Console.ReadLine());
			int count = 0;

			for (int i = 1; i < k; i++)
			{
				for (int j = i + 1; j <= k; j++) {
					if (i % 2 == 0 && j % 2 == 1)
					{
						count++;
					}else if (i % 2 == 1 && j % 2 == 0)
					{
						count++;
					}
				}
			}
			
			Console.WriteLine(count);
		}
	}
}