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;
class P
{
    static void Main()
    {
        string[] nk = Console.ReadLine().Split();
        int n, k;
        n = int.Parse(nk[0]);
        k = int.Parse(nk[1]);
        ulong ans;
        if (k % 2 == 1)
            ans = (ulong)Math.Pow(n / k, 3);
        else
        {
            ans = 0;
            ans += (ulong)Math.Pow(n / k, 3);
            ans += (ulong)Math.Pow(n / (k / 2) - n / k, 3);
        }
        Console.WriteLine(ans);
    }
}