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.Generic;
using System.Linq;
using BitArray = System.Collections.BitArray;
using BigInteger = System.Numerics.BigInteger;

namespace AtCoderProject
{
    public class Program
    {
        public object Calc()
        {
            var line = consoleReader.SplitInt();
            var N = line[0];
            var K = line[1];

            long baisu = N / K;
            long res = baisu * baisu * baisu;

            if (K % 2 == 0)
            {
                var half = (N + K / 2) / K;
                res += half * half * half;
            }

            return res;
        }

        public const int mod = 1000000007;
        #region いつもの
        private readonly ConsoleReader consoleReader;
        public Program() : this(new ConsoleReader()) { }
        public Program(ConsoleReader consoleReader) { this.consoleReader = consoleReader; }
        static void Main() => Console.WriteLine(new Program().Calc());
        public static string MultiLine<T>(IEnumerable<T> source) => string.Join("\n", source);
        #endregion
    }

    #region Console
    public class ConsoleReader { public virtual string String() => Console.ReadLine(); public int Int() => int.Parse(String()); public long Long() => long.Parse(String()); public ulong ULong() => ulong.Parse(String()); public double Double() => double.Parse(String()); public string[] Split() => String().Split().ToArray(); public int[] SplitInt() => String().Split().Select(int.Parse).ToArray(); public long[] SplitLong() => String().Split().Select(long.Parse).ToArray(); public ulong[] SplitULong() => String().Split().Select(ulong.Parse).ToArray(); public double[] SplitDouble() => String().Split().Select(double.Parse).ToArray(); public IEnumerable<string> Repeat(int count) { for (int i = 0; i < count; i++) yield return String(); } public IEnumerable<int> RepeatInt(int count) { for (int i = 0; i < count; i++) yield return Int(); } public IEnumerable<long> RepeatLong(int count) { for (int i = 0; i < count; i++) yield return Long(); } public IEnumerable<string[]> RepeatSplit(int count) { for (int i = 0; i < count; i++) yield return Split(); } public IEnumerable<int[]> RepeatSplitInt(int count) { for (int i = 0; i < count; i++) yield return SplitInt(); } }
    #endregion
}