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_test
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            int output;
            int k = int.Parse(Console.ReadLine());
            if (k % 2 == 0) 
            {
                int K = k / 2;
                output = K * K;
            } else {
                int K = k / 2;
                output = K * (K + 1);
            }
            Console.WriteLine(output);
        }
    }
}