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: Accepted)

using System;
using System.IO;
using System.Linq;
using System.Reflection;
using System.Diagnostics;
using System.Collections.Generic;
using System.Text.RegularExpressions;
using static System.Math;

class P
{
    static void Main()
    {
        int[] nk = Console.ReadLine().Split().Select(int.Parse).ToArray();
        //mod k
        //a
        //b
        //a
        //a+b=k
        //a+a=k
        //全部同じmod
        //kが3の倍数
        long a = nk[0] / nk[1];
        long b = 0;
        if (nk[1] % 2 == 0)
        {
            b = a;
            if (nk[0] - a * nk[1] >= nk[1] / 2)
            {
                b++;
            }
        }
        Console.WriteLine(a * a * a + b * b * b);
    }
}