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)

use proconio::input;
use std::collections::BTreeMap;

fn main() {
    input! {
        n: usize,
        a: [usize; n],
    }

    let mut map = BTreeMap::new();
    for &na in &a {
        let entry = map.entry(na).or_insert(0);
        *entry += 1;
    }

    let mut first = 0;
    let mut second = 0;
    for (key, value) in map.iter().rev() {
        let mut value = *value;
        if first == 0 {
            if value >= 2 {
                first = *key;
                value -= 2;
            }
            if value >= 2 {
                second = *key;
                break;
            }
        } else {
            if value >= 2 {
                second = *key;
                break;
            }
        }
    }
    if first == 0 || second == 0 {
        println!("{}", 0);
    } else {
        println!("{}", first * second);
    }
}