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 maplit::*;
use std::{i32, i64, u32, u64, f64};
use std::cmp::{max, min, Ordering};
use std::collections::*;
use std::io::{stdout, Write, BufWriter};
use std::mem::{swap};
use proconio::marker::{Isize1, Usize1, Bytes, Chars};

#[proconio::fastout]
fn main() {
    proconio::input!{
        n: usize,
        edges: [i64; n],
    }
    let mut cands = BinaryHeap::new();
    let mut ckdup = hashset!();
    for edge in edges {
        if ! ckdup.insert(edge) { cands.push(ckdup.take(&edge).unwrap()); } }
    if cands.len() >= 2 { println!("{}", cands.pop().unwrap() * cands.pop().unwrap()); }
    else { println!("0"); }
}