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 itertools::Itertools;
use proconio::{input, marker::*};

fn main() {
    input! {
        n: usize,
        mut aa: [usize; n]
    };
    aa.sort();
    aa.reverse();

    let mut pre_a = 0;
    let mut count = 0;
    let mut pairs = vec![];
    for a in aa {
        if a == pre_a {
            count += 1;
            if count == 2 {
                pairs.push(a);
                if pairs.len() == 2 {
                    println!("{}", pairs[0] * pairs[1]);
                    return;
                }
                count = 0;
            }
        } else {
            pre_a = a;
            count = 1;
        }
    }
    println!("0");
}