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 9.2.1) (judge verdict: Accepted)

#include <cstdio>
#include <bits/stdc++.h>
#include <algorithm>
#include <iostream>
#include <string>
#include <cmath>
#include <functional>
#include <vector>
#include <stack>
#include <queue>
#include <map>
#define rep(i,n) for (int i=0;i<n;i++)
using ll = long long;
using namespace std;

void solve(){

}
int main (){
    ios::sync_with_stdio(0);
    cin.tie(0);
    ll n,k;
    cin >> n >> k;
    if(k%2==0){
        ll a = n/k;
        ll b;
        if(n>=k/2){
            b = (n-k/2)/k + 1;
        }else{
            b = 0;
        }
        cout << (ll)a*a*a + b*b*b << endl;
    }else{
        int a = n/k;
        cout <<(ll) a*a*a << endl;
    }

    return 0;
}