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 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <stdio.h>

int main() {
  int n, k; long ans = 0;
  scanf("%d%d", &n ,&k);
  if (k == 2) {
    int even = n/2, odd = even + n%2;
    ans += odd + odd*(odd-1)*3 + odd*(odd-1)*(odd-2);
    ans += even + even*(even-1)*3 + even*(even-1)*(even-2);
  } else if (k%2 == 0) {
    int d = n/k;
    ans += d + d*(d-1)*3 + d*(d-1)*(d-2);
    d = n/(k/2);
    d = d/2 + (d%2);
    ans += d + d*(d-1)*3 + d*(d-1)*(d-2);
  } else {
    int d = n/k;
    ans += d + d*(d-1)*3 + d*(d-1)*(d-2);
  }
  printf("%ld\n", ans);
  return 0;
}