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 <bits/stdc++.h>
using namespace std;

//amount of a s.t. a=i(modK)&&a<=N
int fun(int i, int N, int K){
	int x = 0;
	for (int a = 1;a <= N; a++){
		if(a % K == i)
			x++;
	}
	return x;
}

int main(){
	int N, K, ans = 0;
	cin >> N >> K;
	for (int i = 0; i < K; i++){
		if ((2*i) % K == 0)
			ans += fun(i,N,K)*fun(i,N,K)*fun(i,N,K);
	}
	cout << ans << endl;
	return 0;
}