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 <cmath>
#include <string>
using namespace std;

int main(){
    int N,K;
    cin >> N >> K;

    if(N<K/2){
        cout << 0 <<endl;
        return 0;
    }
    int n = 0;
    int k = N;
    while(k >= K){
        k -= K;        
        n++;
    }
    int ans = 0;
    if(K%2 == 1){
        ans = n*n*n;
    }else{
        ans = ((n+1)*2*2*2) + n*n*n;//n==0 -> ans=1
    }
    cout << ans << endl;

}
/*
a+b = kx
b+c = ky
c+a = kz
2(a+b+c)= k(x+y+z)
*/