Problem:
Score : 300 points

Problem Statement

We have N sticks with negligible thickness.
The length of the i-th stick is A_i.

Snuke wants to select four different sticks from these sticks and form a rectangle (including a square), using the sticks as its sides.
Find the maximum possible area of the rectangle.

Constraints

4 \leq N \leq 10^5

1 \leq A_i \leq 10^9

A_i is an integer.

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 ... A_N

Output

Print the maximum possible area of the rectangle.
If no rectangle can be formed, print 0.

Sample Input 1

6
3 1 2 4 2 1

Sample Output 1

2

1 \times 2 rectangle can be formed.

Sample Input 2

4
1 2 3 4

Sample Output 2

0

No rectangle can be formed.

Sample Input 3

10
3 3 3 3 4 4 4 5 5 5

Sample Output 3

20

Solution in Rust (1.15.1) (judge verdict: Accepted)

use std::error::Error;
use std::io;

fn main() {
    if let Err(e) = run() {
        println!("{}", e);
    }
}

fn run() -> Result<(), Box<Error>> {
    io::stdin().read_line(&mut String::new())?;
    let mut s = String::new();
    io::stdin().read_line(&mut s)?;
    let mut a: Vec<u64> = s.trim().split(" ").map(|x| x.parse().expect("")).collect();
    a.sort();
    a.reverse();
    let mut n = 0;
    let mut pre = 0;
    let mut ans = 0;
    for x in a {
        if pre == x {
            n += 1;
            if n == 2 {
                if ans == 0 {
                    ans = x;
                } else {
                    println!("{}", ans * x);
                    return Ok(());
                }
                n = 0;
            }
            continue;
        }
        pre = x;
        n = 1;
    }
    if n == 2 && ans != 0 {
        println!("{}", ans * pre);
        return Ok(());
    }
    println!("0");
    Ok(())
}