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::io::Read;
use std::collections::HashMap;

fn main() {
    let mut buf = String::new();
    std::io::stdin().read_to_string(&mut buf).unwrap();

    let answer = solve(&buf);

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

fn solve(input: &str) -> String {
    let mut iterator = input.split_whitespace();

    let n: usize = iterator.next().unwrap().parse().unwrap();

    let mut counts: HashMap<usize, usize> = HashMap::new();
    for _ in 0..n {
        let ai: usize = iterator.next().unwrap().parse().unwrap();
        *counts.entry(ai).or_insert(0) += 1;
    }

    let mut keys: Vec<&usize> = counts.keys().collect::<Vec<_>>();
    keys.sort();

    let mut edge1: usize = 0;
    for &&key in keys.iter().rev() {
        if edge1 == 0 {
            if counts[&key] >= 4 {
                return format!("{}", key * key);
            } else if counts[&key] >= 2 {
                edge1 = key;
            }
        } else {
            if counts[&key] >= 2 {
                return format!("{}", edge1 * key);
            }
        }
    }

    return "0".into();
}