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 <map>
#include <algorithm>
#include <string>
#include <vector> 
#include <numeric>
#include <cmath>


#define CHMAX(a,b) a = std::max(a,b)
#define CHMIN(a,b) a = std::min(a,b)
#define CHABS(a) a = std::abs(a) 
#define COUT(a) std::cout << a << std::endl
#define CERR(a) std::cerr << a << std::endl

using namespace std;
using lli = long long int;

lli mod197 = 1000000007LL;

// ax + by = gcd(a,b) 最大公約数
template< typename T >
T extgcd(T a, T b, T& x, T& y) {
	T d = a;
	if (b != 0) {
		d = extgcd(b, a % b, y, x);
		y -= (a / b) * x;
	}
	else {
		x = 1;
		y = 0;
	}
	return d;
}

int main(void) {

	lli N, K;
	cin >> N >> K;

	if (K % 2 == 0) {
		COUT((lli)pow((lli)(N / K), 3) + pow((lli)(((N - K / 2) / K) + 1), 3));
	}
	else {
		COUT((lli)pow((lli)(N / K),3));
	}

	return 0;
}