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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include<iostream>
#include<vector>
using namespace std;
int main(){
	long long a,n,k,res=0;
	cin>>n>>k;
	if(k%2==0){
		long long i=n/k;
		long long j=k-j;
//		cout<<i<<" "<<j<<endl;
		res+=i;
		if(i>=1){
			res+=i*(i-1)*3;
		}
		if(i>=2){
			res+=i*(i-1)*(i-2);
		}
		res+=j;
		if(j>=1){
			res+=j*(j-1)*3;
		}
		if(j>2){
			res+=j*(j-1)*(j-2);
		}
	}else{
		long long i=n/k;
//		cout<<i<<endl;
		res+=i;
		if(i>=1){
			res+=i*(i-1)*3;
		}
		if(i>=2){
			res+=i*(i-1)*(i-2);
		}
	}
	cout<<res;
	return 0;
}