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 n,k,ans1,ans2=0,i,ans3=0;
	scanf("%lld%lld",&n,&k);
	if(k%2==0){
		ans1=n*2/k;
		if(ans1%2==0){
			ans2=(ans1/2)*(ans1/2-1)*6;
			ans3=(ans1/2)*(ans1/2-1)*(ans1/2-2)*2;
		}
		else{
			ans2=((ans1/2+1)*(ans1/2)+(ans1/2)*(ans1/2-1))*3;
			ans3=(ans1/2+1)*(ans1/2)*(ans1/2-1)+(ans1/2)*(ans1/2-1)*(ans1/2-2);
		}
	}
	else{
		ans1=n/k;
		ans2=ans1*(ans1-1)*3;
		ans3=ans1*(ans1-1)*(ans1-2);
	}
	printf("%lld",ans1+ans2+ans3);
	return 0;
}