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: Accepted)

#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using ull = unsigned long long;
using ld = long double;
#define FOR(NAME, START, NUM) for (ll NAME = START; NAME < (NUM); NAME++)
#define REP(NAME, NUM) for (ll NAME = 0; NAME < (NUM); NAME++)
#define BREP(NAME, NUM) for (ll NAME = (NUM)-1; NAME >= 0; NAME--)
#define ALL(NAME) (NAME).begin(), (NAME).end()
const ll INF =1000000000000000;

int main(){
  ll n,k; cin >> n >> k;
  
  ll ans=pow(n/k,3);
  
  if(k%2 == 0){
    ans+=pow(n/(k/2)-n/k,3);
  }
  
  cout << ans << endl;
}