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 Rust (1.42.0) (judge verdict: Runtime Error)

use proconio::{fastout, input};

#[fastout]
fn main() {
    input! {
        n: usize,
        k: usize,
    }

    let mut v = vec![0; n + 1];
    for i in 1..=n {
        v[i % k] += 1;
    }

    let mut ans: i128 = v[0] * v[0] * v[0];
    if k % 2 == 0 {
        ans += v[k / 2] * v[k / 2] * v[k / 2];
    }

    println!("{}", ans);
}