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;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AABC
{
    class Program
    {
        static void Main(string[] args)
        {
            int k = int.Parse(Console.ReadLine());
            int gu = 0;
            int ki = 0;

            if(k%2==0)//偶数なら
            {
                gu = k / 2;
                ki = gu;
            }
            else//奇数
            {
                gu = (k - 1) / 2;
                ki = k - gu;
            }

            Console.WriteLine(gu*ki);

        }
    }
}