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 (1.15.1) (judge verdict: Accepted)

use std::io::Read;

fn main() {
    let mut buf = String::new();
    std::io::stdin().read_to_string(&mut buf).unwrap();
    let mut iter = buf.split_whitespace();
    let n: i64 = iter.next().unwrap().parse().unwrap();
    let k: i64 = iter.next().unwrap().parse().unwrap();
    let mut c: u64 = 0;
    let mut c2: u64 = 0;
    if k % 2 != 0 {
        for a in 1..(n + 1) {
            if a % k == 0 {
                c += 1;
            }
        }
        println!("{}", c.pow(3));
    } else {
        for a in 1..(n + 1) {
            if a % k == 0 {
                c += 1;
            } else if a % k == k / 2 {
                c2 += 1;
            }
        }
        println!("{}", c.pow(3) + c2.pow(3));
    }
}