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 Program
{

    static void Main(string[] args)
    {
        string[] str = Console.ReadLine().Split(' ');
        int n = int.Parse(str[0]);
        int k = int.Parse(str[1]);
        ulong res = 0;
        //Console.WriteLine("{0}, {1}", n, k);
        if (k % 2 == 0)
        {
            int kk = k / 2;
            int knum = n / k;
            int kknum = n / kk - knum;
            res = powi(knum, 3) + powi(kknum, 3);
        }
        else
        {
            int knum = n / k;
            res = powi(knum, 3);
        }
        Console.WriteLine("{0}", res);
    }

    static ulong powi(int x, int n)
    {
        ulong res = 1;
        for (int i = 0; i < n; i++)
        {
            res *= (ulong)x;
        }
        return res;
    }
}