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 (1.42.0) (judge verdict: Accepted)

#[allow(unused_imports)]
use proconio::marker::{Bytes, Chars, Usize1};
use proconio::{fastout, input};
#[fastout]
fn main() {
    input! {
        n: usize,
        mut a: [usize; n]
    }
    let mut pair = vec![0; n];
    a.sort();
    let mut i = n;
    let mut skip = false;
    while i > 1 {
        if skip {
            skip = false;
            i -= 1;
            continue;
        }
        if a[i - 1] == a[i - 2] {
            if pair[0] == 0 {
                pair[0] = a[i - 2];
            } else {
                pair[1] = a[i - 2];
                break;
            }
            skip = true;
        }
        i -= 1;
    }
    println!("{}", pair[0] * pair[1]);
}