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 n,k;
	scanf("%lld %lld",&n,&k);
	long long ans=0;
	long long i;
	long long temp = 0;
	
	//kが奇数のとき
	if( k%2 != 0){
		temp = n/k;
		ans = temp * temp * temp;
	}else{
		long long a = n/k;
		long long b = 0;
		for(i=1;i<=n;i++){
			if( (i%k) ==(k/2)){
				b++;
			}
		}
		ans = a*a*a + b*b*b;
	}
	printf("%lld",ans);
	
	return 0;
}