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: Accepted)

#include <stdio.h>
int main(void){
 long long int a,b,c,d;
 scanf("%lld%lld",&a,&b);
 c=a/b;
 d=(a/(b/2))-c;
 if(b%2==1){
  if(c==0) printf("0");
  else if(c==1) printf("1");
  else printf("%lld",c+c*(c-1)*3+c*(c-1)*(c-2));
 }else{
  if(d==0) printf("0");
  else if(d==1&&c==0) printf("1");
  else if(d==1&&c==1) printf("2");
  else if(d==2&&c==1) printf("9");
  else printf("%lld",c+c*(c-1)*3+c*(c-1)*(c-2)+d+d*(d-1)*3+d*(d-1)*(d-2));
 }
return 0;
}