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

public class Remainder
{
    static long N;
    static long K;
    static public void Main ()
    {
        var NK = Console.ReadLine().Split(' ').Select(x => long.Parse(x)).ToArray();
        N = NK[0];
        K = NK[1];
        
        var ans = 0L;
        //aのKの余りによって場合分け
        for(int i = 0;i < K;i++){
            //aの余りと足してKの倍数になる数字
            var comp = (K-i)%K;
            if((comp*2)%K == 0){
                //あまりが一緒の積を計算
                ans += CheckModP(i)*CheckModP(comp)*CheckModP(comp);
            }
            
        }
        
        Console.WriteLine(ans);

    }
    
    //Pは0以上K-1以下の値
    static long CheckModP(long P){
        var res = N/K;
        if(P != 0 && (N%K) >= P){
            res++;
        }
        return res;
    }

}