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 <string>
#include <iostream>
#include <vector>
 
int main(){
  std::string strN, strK;
  long long N, K;
  
  std::cin >> strN;
  std::cin >> strK;
  N = std::stol(strN);
  K = std::stol(strK);
  
  long long counter_3 = 0;
  // a > b > cを家庭
  
  // Kの倍数の数を調べる
  long long countK = 0;
  for (long long a=1; a<=N; a++){
      if (a % K == 0)
	      countK++;
  }
  
    	counter_3 += countK * countK * countK;
 
  
  countK = 0;
  if (K%2 == 0){
    for (long long a=1; a<=N; a++){
      if (a % K == K/2)
	      countK++;
    }
    
    	counter_3 += countK * countK * countK;
  }
 
  
  std::cout << counter_3 << std::flush;
}