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 <math.h>
#include <string>
#include <algorithm>
#include <ctype.h>
#include <vector>
#include <numeric>
using namespace std;

#define ll long long int
#define rep(i, a, n) for (int i = a; i < n; i++)
#define INF 1e9
#define LINF 1e17
#define MOD (int)(1e9 + 7)
#define pi 3.141592653589
#define pii pair<int, int>
#define vi vector<int>
#define vll vector<ll>
#define ALL(a) (a).begin(), (a).end()
#define sort_v(a) sort(a.begin(), a.end())
#define fi first
#define se second

void print(bool a)
{
    if (a == true)
        cout << "Yes" << endl;
    else
        cout << "No" << endl;
}

ll gcd(ll a, ll b)
{
    while (1)
    {
        if (a < b)
            swap(a, b);
        if (b == 0)
            break;
        a %= b;
    }
    return a;
}

int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

int main()
{
    ll n, k;
    cin >> n >> k;

    ll cnt0 = n / k;
    ll cntk = (n + k / 2) / k;

    if (k % 2 == 0)
    {
        cout << powl(cnt0, 3) + powl(cntk, 3) << endl;
    }
    else
    {
        cout << powl(cnt0, 3) << endl;
    }
}