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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <iostream>
using namespace std;

int main (void){
	long long int n,k,i,v=0;
	cin >> n >>k;
	v+=((n-n%k)/k)*((n-n%k)/k)*((n-n%k)/k);
	if(k%2==0){
		
		k/=2;
		v+=((((n-n%k)/k)+((n-n%k)/k)%2)/2)*((((n-n%k)/k)+((n-n%k)/k)%2)/2)*((((n-n%k)/k)+((n-n%k)/k)%2)/2);
	}
	cout << v << endl;
	return 0;
}