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[] s = Console.ReadLine().Split().ToArray();
        int n = int.Parse(s[0]);
        int k = int.Parse(s[1]);

        long ans = 0;

        if (k % 2 == 1)
        {
            long tmp = 0;
            for (int i = 1; i <= n; i++)
            {
                if (i % k == 0) tmp++;
            }
            ans = tmp * tmp * tmp;
        }
        else
        {
            long tmp = 0;
            long tmp2 = 0;
            for (int i = 1; i <= n; i++)
            {
                if (i % k == 0) tmp++;
                else if (i % k == k / 2) tmp2++;
            }
            ans = tmp * tmp * tmp + tmp2 * tmp2 * tmp2;
        }

        Console.WriteLine(ans);
    }
}