Problem:
Score : 300 points

Problem Statement

You are given integers N and K. Find the number of triples (a,b,c) of positive integers not greater than N such that a+b,b+c and c+a are all multiples of K.
The order of a,b,c does matter, and some of them can be the same.

Constraints

1 \leq N,K \leq 2\times 10^5

N and K are integers.

Input

Input is given from Standard Input in the following format:

N K

Output

Print the number of triples (a,b,c) of positive integers not greater than N such that a+b,b+c and c+a are all multiples of K.

Sample Input 1

3 2

Sample Output 1

9

(1,1,1),(1,1,3),(1,3,1),(1,3,3),(2,2,2),(3,1,1),(3,1,3),(3,3,1) and (3,3,3) satisfy the condition.

Sample Input 2

5 3

Sample Output 2

1

Sample Input 3

31415 9265

Sample Output 3

27

Sample Input 4

35897 932

Sample Output 4

114191

Solution in C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

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

namespace abc108_c
{
    class program
    {
        static void Main(string[] args)
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = input[0];
            int K = input[1];
            int ans = 0;
            if (K%2==0)
            {
                K = K/2;
                for (int i = 1; i <= N / K; i++)
                {
                    ans += (Change_downint((i+N/K)/2) - Change_upint((i+1/K)/2) + 1) * (Change_downint((i+N/K)/2) - Change_upint((i+1/K)/2) + 1);
                }
            }else
            {   
                for (int i = 1; i <= N / K; i++)
                {
                    ans += (N/K-1/K+1) * (N/K-1/K+1);
                }
            }
            Console.WriteLine(ans);
        }

        static int Change_upint(double n)
        {
            if (Validation.IsDecimal(n)) {
                int ans = (int)n+1;
                return ans;
            }else
            {
                int ans = (int)n;
                return ans;
            }
        }

        static int Change_downint(double n)
        {
            if (Validation.IsDecimal(n)) {
                int ans = (int)n;
                return ans;
            }else
            {
                int ans = (int)n;
                return ans;
            }
        }
        
        public sealed class Validation {

        #region　IsDecimal メソッド

            /// ------------------------------------------------------------------------------
            /// <summary>
            ///     指定した値が小数を含むかどうかを返します。</summary>
            /// <param name="dValue">
            ///     検査対象となる値。</param>
            /// <returns>
            ///     小数を含む場合は true。それ以外は false。</returns>
            /// ------------------------------------------------------------------------------
            public static bool IsDecimal(double dValue) {
                if (dValue - System.Math.Floor(dValue) != 0) {
                    return true;
                }

                return false;
            }

        #endregion

        }
   }
}