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.Collections.Generic;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        var inputs = Console.ReadLine().Split(' ');

        var N = long.Parse(inputs[0]);
        var K = long.Parse(inputs[1]);

        long num = 0;

        if (K % 2 == 0)
        {
            long harf = 0;

            for (var i = 1; i < N + 1; i++)
            {
                if (i % K == K / 2)
                {
                    harf++;
                }
            }

            if (harf != 0)
            {
                if (N >= K)
                {
                    num = (long)Math.Pow((double)(N / K), 3) + (long)Math.Pow((double)harf, 3);
                }
                else
                {
                    num = (long)Math.Pow((double)harf, 3);
                }
            }
        }
        else
        {
            if (N >= K)
            {
                num = (long)Math.Pow((double)(N / K), 3);
            }
        }

        Console.WriteLine(num);
        Console.Read();
    }
}