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;

int main(){
  
  long long n,k;
  cin >>n>>k;
  
  long long ans=0;
 
  //kで割り切れるa,b,cの種類
  long long num1=n/k; 
  
  //kで割った余りがk/2になるa,b,cの種類
  long long num2=0;
  if(k%2==0){
    num2=(n+(k/2))/k;
  }
  ans=num1*num1*num1+num2*num2*num2;
  // cout <<"num1:"<<num1<<endl;
  // cout <<"num2:"<<num2<<endl;
  cout <<ans<<endl;
  return 0;
}