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;

namespace CSharpSample01
{

    

    class Program
    {

        static int N;
        static int K;

        static void Main(string[] args)
        {
            string[] strer = Console.ReadLine().Split(' ');
            N = int.Parse(strer[0]);
            K = int.Parse(strer[1]);

            int one = 0;
            int half = 0;

            for(int i = 1; i <= N; i++)
            {
                if(i % K == 0)
                {
                    one++;
                }
                else if(i % K == K / 2)
                {
                    half++;
                }
            }

            long ans = 0;

            if(K % 2 != 0)
            {
                ans += (long)Math.Pow(one, 3);
            }
            else
            {
                ans += (long)(Math.Pow(one, 3) + Math.Pow(half, 3));
            }

            Console.WriteLine(ans);

        }
    }
}