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;

namespace easy_Atcoder
{
    class Program
    {
        static void Main(string[] args)
        {
            int K = int.Parse(Console.ReadLine());
            int oddNum = K/2;
            int evenNum = K - oddNum;
            Console.WriteLine(oddNum*evenNum);
        }   
            // float[] ABN = Array.ConvertAll(Console.ReadLine().Split(),float.Parse);
            // long[] NK = Array.ConvertAll(Console.ReadLine().Split(),long.Parse);
            // long N = NK[0];
            // long K = NK[1];
            // long K = long.Parse(Console.ReadLine());
            // int[] abc = Array.ConvertAll(Console.ReadLine().Split(),int.Parse);
            // string S = Console.ReadLine();
            // int x = int.Parse(Console.ReadLine());
            // int b = int.Parse(Console.ReadLine());
            // int N = int.Parse(Console.ReadLine());
            // float N = float.Parse(Console.ReadLine());
            // string c = Console.ReadLine();
            // string B = Console.ReadLine();
            // string[] s = Console.ReadLine().Split();
            // var s = (new string[] { Console.ReadLine(), Console.ReadLine(), Console.ReadLine() }).Select(float.Parse);
            // float[] a = Array.ConvertAll(Console.ReadLine().Split(),float.Parse);
            // var X = Console.ReadLine();
            // int[,] abcd = new int[NMQ[2],4];
            // int[] A = new int[NMQ[0]];
            // int[] A_num = new int[NMQ[0]];
            // long[] s=Array.ConvertAll(Console.ReadLine().Split(),long.Parse);
            // var hs=new Dictionary<long,long>();
            // for(int i=0;i<s[0];i++){
            //     long[] q=Array.ConvertAll(Console.ReadLine().Split(),long.Parse);
            //     if(hs.ContainsKey(q[0])){hs[q[0]]+=q[1];}
            //     else{hs.Add(q[0],q[1]);}
            // }
        
        // //最小公倍数
        // static long Lcm(long a, long b)
        // {
        //     return a/Gcd(a,b)*b;
        // }

        // //最大公約数
        // static long Gcd(long m, long n)
        // {
        //     if(n==0)return m;
        //     return Gcd(n,m%n);
        // }
    }
}