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: Wrong Answer)

#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>
#include <string>
#include <map>
#include <cstdio>
#include <iomanip>
#include <list>
#include <set>

using namespace std;

int main(void) {
	int N, K;
	long long ans = 0;
	scanf("%d %d\n", &N, &K);

	if (K % 2 == 0) {
		ans = (N / K)*(N / K)*(N / K);
		long long b = (N / K) + (N%K >= K/2 ? 1 : 0);
		ans += b * b * b;
	}
	else {
		ans = (N / K)*(N / K)*(N / K);
	}
	
	printf("%lld\n", ans);

	return 0;
}