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;
#[allow(unused_imports)]
use proconio::marker::{Bytes, Chars};
#[allow(unused_imports)]
use std::cmp::{min, max};
use std::collections::HashMap;

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

	let mut h = HashMap::new();

	for i in 0..a.len() {
		let count = h.entry(a[i]).or_insert(0);
		*count += 1;
	}

	a.sort_by_key(|&x| std::cmp::Reverse(x));
	a.dedup();
	let mut t = 0usize;
	let mut ans = [0; 2];
	for i in 0..a.len() {
		if h.get(&a[i]).unwrap() >= &4 && t == 0 {
			println!("{}", a[i]*a[i]);
			return;
		}

		if h.get(&a[i]).unwrap() >= &2 {
			ans[t] = a[i];
			t += 1;
			if t == 2 {
				println!("{}", ans[0]*ans[1]);
				return;
			}
		}
	}
	println!("0");
}