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;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp90 {
    class Program {
        static void Main(string[] args) {
            var st = Console.ReadLine().Split().Select(int.Parse).ToArray();
            int N = st[0];
            int K = st[1];

            long ans = 0;

            if (K == 1) {
                ans = (long)Math.Pow(N, 3);
                Console.WriteLine(ans);
                return;
            }

            if (K == 2) {
                ans = (long)Math.Pow(N / 2, 3) + (long)Math.Pow(N - (N / 2), 3);
                Console.WriteLine(ans);
                return;
            }

            if (K % 2 == 1) {
                ans = (long)Math.Pow(N / K, 3);
                Console.WriteLine(ans);
                return;
            } else {
                int l = K / 2;

                //全部偶数のパターンを足す
                ans += (long)Math.Pow(N / K, 3);

                //全部奇数のパターンを足す
                ans += (long)Math.Pow((N / l) - (N / K), 3);

                Console.WriteLine(ans);
                return;
            }
        }

    }
}