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 (1.15.1) (judge verdict: Accepted)

use std::io;
use std::collections::BTreeMap;


/// https://beta.atcoder.jp/contests/arc081/tasks/arc081_a
fn main() {
    let _n = read_one::<usize>();
    let an = read::<isize>();
    let mut bm = BTreeMap::new();

    for a in an {
        *bm.entry(a).or_insert(0) += 1;
    }

    let mut vec = bm.into_iter().filter(|&(_, v)| v >= 2).collect::<Vec<_>>();
    vec.sort_by_key(|&(k, _)| -k);
    if vec.len() < 2 { println!("0"); }
    else if vec[0].1 >= 4 { println!("{}", vec[0].0 * vec[0].0); }
    else { println!("{}", vec[0].0 * vec[1].0)}
}


#[allow(dead_code)]
fn read<T>() -> Vec<T>
where T:
    std::str::FromStr,
    T::Err: std::fmt::Debug {

    let mut buf = String::new();
    io::stdin().read_line(&mut buf).unwrap();
    buf.split_whitespace()
        .map(|s| s.trim().parse().unwrap())
        .collect()
}

#[allow(dead_code)]
fn read_one<T>() -> T
where T:
    std::str::FromStr,
    T::Err: std::fmt::Debug {

    let mut buf = String::new();
    io::stdin().read_line(&mut buf).unwrap();
    buf.trim().parse().unwrap()
}