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;
#define rep(i,n) for(int i = 0; i < n; i++)
typedef long long ll;
typedef pair<int, int> PI;
typedef pair<ll, ll> PL;
using graph = vector<vector<int>>;

const double pi = 3.14159265358979323846;
const ll mod = 1000000007;


int main() {
	cin.tie(0);
   	ios::sync_with_stdio(false);
	int n, k; cin >> n >> k;
	ll ans = 0;
	ll tmp = n/k;
	ans += tmp * tmp * tmp;
	if(k % 2 == 0){
		vector<int> res(k, 0);
		for(int i = 1; i <= n; i++){
			res.at(i % k)++;
		}
		for(int i = 1; i < k; i++){ //一つの数のあまりがiの時
			if(k - 2 * i == 0){
				ans += res.at(i) * res.at(k-i) * res.at(k-i);
			}
		}
	}
	cout << ans << endl;
}