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)

fn read<T: std::str::FromStr>() -> Vec<T> {
    let mut buf = String::new();
    std::io::stdin().read_line(&mut buf).ok();
    buf.split_whitespace()
        .map(|x| x.parse().ok().unwrap())
        .collect()
}

fn main() {
    let _n: usize = read()[0];
    let mut a: Vec<i64> = read();
    a.sort_by_key(|&x| -x);
    let mut pre = 0;
    let mut l1 = 0;
    let mut l2 = 0;
    for &x in a.iter() {
        if pre == 0 || pre != x {
            pre = x;
            continue;
        }

        if l1 == 0 {
            l1 = x;
            pre = 0;
        } else {
            l2 = x;
            break;
        }
    }
    println!("{}", l1 * l2);
}