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.IO;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;

namespace AtCoder
{
	public class ABC
	{
		static readonly int mod = 1000000007;  // 10^9+7

		static void Main(string[] args)
		{
			Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });


			int k = int.Parse(Console.ReadLine());
			//string s = Console.ReadLine();
			//int[] vs = Console.ReadLine().Split().Select(int.Parse).ToArray();
			//long[] a = Console.ReadLine().Split().Select(long.Parse).ToArray();
			int odd = k / 2 + k % 2;
			int even = k / 2;
			//System.Diagnostics.Debug.WriteLine($"{odd} {even}");
			Console.WriteLine(odd * even);


			Console.Out.Flush();
		}
	}
}