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.Linq;
using System.Text.RegularExpressions;

namespace abc108_c
{
    class program
    {
        static void Main(string[] args)
        {
            int[] input = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = input[0];
            int K = input[1];
            long ans = 0;
            if (K%2==0)
            {
                K /= 2;
                for (int i = 1; i <= N / K; i++)
                {
                    double max = ((double)i+(double)N/(double)K)/2;
                    double min = ((double)i+1/(double)K)/2;
                    ans += ((int)Math.Floor(max) - (int)Math.Ceiling(min) + 1) * ((int)Math.Floor(max) - (int)Math.Ceiling(min) + 1);
                }
            }else
            {   
                for (int i = 1; i <= N / K; i++)
                {
                    double max = (double)i+(double)N/(double)K;
                    double min = (double)i+1/(double)K;
                    ans += ((int)Math.Floor(max) - (int)Math.Ceiling(min) + 1) * ((int)Math.Floor(max) - (int)Math.Ceiling(min) + 1);
                }
            }
            Console.WriteLine(ans);
        }
   }
}