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.Linq;
using System.Collections;
using System.Collections.Generic;
using static System.Console;
class Program
{
    static void Main()
    {
        //入力
        //int[] input = Console.ReadLine().Split().Select(i => int.Parse(i)).ToArray();
        //string input= Console.ReadLine();
        int input =int.Parse(Console.ReadLine());
        Console.WriteLine(Math.Ceiling((double)input/2)*Math.Floor((double)input/2));


        /*
        //処理速度計測
        var sw = new System.Diagnostics.Stopwatch();
        sw.Start();
        //解法
 
        //処理速度表示
        sw.Stop();
        TimeSpan ts = sw.Elapsed;
        Console.WriteLine((int)(Math.Floor((ts).TotalMilliseconds))+"ms");
        */
        Console.ReadLine();

    }
}