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 "bits/stdc++.h"
#define FOR(i, a, b) for (int i = a; i < b; i++)
#define mFOR(i, a, b) for (int i = a; i > b; i--)
#define MP make_pair
#define PB push_back
#define ALL(v) v.begin(), v.end()
#define N 100007
#define INF 1000000007
using namespace std;

typedef long long ll;

ll gcd(ll a, ll b) { return b == 0 ? a : gcd(b, a % b); }
ll fceil(ll a, ll b) { return (a % b == 0 ? a / b : a / b + 1); }
int main()
{
   ll n, k;
   cin >> n >> k;
   ll n1 = n / k, n2 = (n - k / 2) / k + 1;
   if (k % 2 != 0)
   {
      cout << pow(n1, 3LL) << endl;
   }
   else
   {
      cout << pow(n1, 3LL) + pow(n2, 3LL) << endl;
   }

   return 0;
}