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)

#pragma warning disable

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.Serialization.Formatters.Binary;
using System.Text;
using System.Diagnostics;
using System.Numerics;
using System.Collections;

static class MainClass
{
    public static void Main()
    {
        var n = Console.ReadLine().ToInt32();
        var kisu = n % 2 == 0;
        Console.WriteLine(kisu ? n*n / 4 : (n*n + 1)/ 4);
        Console.ReadLine();
    }


    #region ライブラリ
    #region 二分探索
    public static int LowerBound<T>(this IEnumerable<T> ar, int start, int end, T value, IComparer<T> comparer)
    {
        var arr = ar.ToArray();
        int low = start;
        int high = end;
        int mid;
        while (low < high)
        {
            mid = ((high - low) >> 1) + low;
            if (comparer.Compare(arr[mid], value) < 0)
                low = mid + 1;
            else
                high = mid;
        }
        return low;
    }

    public static int LowerBound<T>(this IEnumerable<T> arr, T value) where T : IComparable
    {
        return LowerBound(arr, 0, arr.Count(), value, Comparer<T>.Default);
    }

    public static int UpperBound<T>(this IEnumerable<T> ar, int start, int end, T value, IComparer<T> comparer)
    {
        var arr = ar.ToArray();
        int low = start;
        int high = end;
        int mid;
        while (low < high)
        {
            mid = ((high - low) >> 1) + low;
            if (comparer.Compare(arr[mid], value) <= 0)
                low = mid + 1;
            else
                high = mid;
        }
        return low;
    }

    public static int UpperBound<T>(this IEnumerable<T> arr, T value)
    {
        return UpperBound(arr, 0, arr.Count(), value, Comparer<T>.Default);
    }
    #endregion
    #region　bit全探索
    /// <summary>
    /// Bit全探索用のライブラリ
    /// </summary>
    /// <param name="len">作る配列の長さ(lenが0以下だと（ｒｙ)</param>
    /// <returns>できた配列</returns>
    public static List<List<bool>> CreateBits(int len)
    {
        return CreateBitsPri(new List<List<bool>>(), len, 0);
    }

    private static List<List<bool>> CreateBitsPri(List<List<bool>> bits, int len, int count)
    {
        if (count == 0)
        {
            var lss = new List<bool>();
            lss.Add(true);
            bits.Add(lss);
            var lx = new List<bool>();
            lx.Add(false);
            bits.Add(lx);
            count++;
            return CreateBitsPri(bits, len, count);
        }
        if (len - count == 0)
            return bits;
        count++;
        var ls = new List<List<bool>>();
        foreach (var item in bits)
        {
            var x = item.ToList();
            x.Add(true);
            item.Add(false);
            ls.Add(item);
            ls.Add(x);
        }
        return CreateBitsPri(ls, len, count);
    }
    #endregion
    #region 文字列処理

    public static int ToInt32(this string str)
    {
        return int.Parse(str);
    }
    public static List<string> SplittedStringToList(this string str)
    {
        return str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries)
            .ToList();
    }
    public static List<int> SplittedStringToInt32List(this string str)
    {
        return str.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(x => int.Parse(x)).ToList();
    }
    #endregion
    #region エラトステネスの篩
    /**
     * MIT License

Copyright (c) 2018 gushwell

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
     **/
    public class BoundedBoolArray
    {
        private BitArray _array;
        private int _lower;

        public BoundedBoolArray(int lower, int upper)
        {
            _array = new BitArray(upper - lower + 1);
            _lower = lower;
        }

        public bool this[int index]
        {
            get
            {
                return _array[index - _lower];
            }
            set
            {
                _array[index - _lower] = value;
            }
        }
    }
    public static IEnumerable<int> GeneratePrimes()
    {
            var primes = new List<int>() { 2, 3 };
            foreach (var p in primes)
                yield return p;
            int ix = 0;
            while (true)
            {
                int prime1st = primes[ix];
                int prime2nd = primes[++ix];
                var lower = prime1st * prime1st;
                var upper = prime2nd * prime2nd - 1;
                var sieve = new BoundedBoolArray(lower, upper);
                foreach (var prime in primes.Take(ix))
                {
                    var start = (int)Math.Ceiling((double)lower / prime) * prime;
                    for (int index = start; index <= upper; index += prime)
                        sieve[index] = true;
                }
                for (int i = lower; i <= upper; i++)
                {
                    if (sieve[i] == false)
                    {
                        primes.Add(i);
                        yield return i;
                    }
                }
        }
    }
    #endregion
    #region DeepClone
    public static T DeepClone<T>(this T src)
    {
        using (var memoryStream = new MemoryStream())
        {
            var binaryFormatter
              = new BinaryFormatter();
            binaryFormatter.Serialize(memoryStream, src);
            memoryStream.Seek(0, SeekOrigin.Begin);
            return (T)binaryFormatter.Deserialize(memoryStream);
        }
    }
    #endregion
    #region 順列を作るやつ
    public static List<List<int>> GeneratePermutations(int n)
    {
        if (n > 14)
            throw new ArgumentOutOfRangeException();
        var ls = new List<List<int>>();
        ls.Add(new List<int>() { 1 });

        for (int i = 0; i < n - 1; i++)
        {
            var newls = new List<List<int>>();
            ls.ForEach(x =>
            {
                var len = x.Count;
                for (int j = 0; j <= len; j++)
                {
                    var item = x.DeepClone();
                    item.Insert(j, i + 2);
                    newls.Add(item);
                }
            });
            ls = newls;
        }
        return ls;

    }
    #endregion
    #endregion
}