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 (Clang 3.8.0) (judge verdict: Accepted)

#include <stdio.h>

int main(){
  int n,k;
  scanf("%d%d",&n,&k);
  int i = 0;
  long long int ans=0;
  for(i=1;i<=k;i++){
    if((i+i)%k == 0 && i <= n){
      long long int num = (n-i)/k+1;
      ans = ans + num*num*num;
    }
  }
  printf("%lld\n",ans);
  return 0;
}