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: Wrong Answer)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;

namespace kp
{
    public class Program
    {
        //static long[,] cmb;
        static int MAX_INT = 1000000007;
        static void Main(string[] args)
        {
            //long N = long.Parse(Console.ReadLine());
            int[] input = Console.ReadLine().Split().Select(p => int.Parse(p)).ToArray();
            int N = input[0];
            int K = input[1];

            long ret = 0;
            for (int a = 1; a <= N; a++ )
            {
                // aを決めたとき、b=?K-a, c=?K-a→b+c = ?K-2aなので
                if((2*a)%K != 0)
                {
                    // a,b,cが取れない
                    continue;
                }
								//Console.WriteLine(a + " ");
                // b, cの取り方は・・・1 <= minK-a～maxK-a <= Nまでなので
                int min = (1 + a) / K;
                if ((1 + a) % K != 0) min++;

                int max = (N + a) / K;
                int bc = max - min + 1;

                if (bc <= 0) continue;

                // b,cそれぞれなりうるから
                ret += bc * bc;
                //ret %= MAX_INT;
            }
            Console.WriteLine(ret + "");
        }
    }
}