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>
using namespace std;
#pragma GCC optimize("Ofast")
#define rep(i,n) for(int i=0;i<n;i++)
const long long MOD = 1000000007;
typedef pair<int,int> P;
typedef long long ll;
ll n,m,k,ans;
string s;

int main(){
  cin >> n >> k;
  ll a = 0;
  ll b = 0;
  for(int i = 1; i<=n; i++) {
    if(i % k == 0) a++;
    if(i % k == k/2) b++;
  }
  if(k%2){
    cout << pow(a,3) << "\n";
  }else{
    cout << pow(a,3)+pow(b,3) << "\n";
  }
}