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: Runtime Error)

using System;
using System.Linq;

namespace abc108_b
{
    class program
    {
        static void Main(string[] args)
        {
            string[] input = Console.ReadLine().Split(' ');
            int x1 = int.Parse(input[0]);
            int y1 = int.Parse(input[1]);
            int x2 = int.Parse(input[2]);
            int y2 = int.Parse(input[3]);
            int x3 = x2 + y1 - y2;
            int x4 = x3 - x2 + x1;
            int y3 = y2 + x2 - x1;
            int y4 = y3 + y1 - y2;
            Console.WriteLine(x3 + " " + y3 + " " + x4 + " " + y4);
        }
    }
}