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 std::collections::BTreeMap;
use std::cmp::max;
use std::io::*;
use std::str::FromStr;

fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin
        .bytes()
        .map(|c| c.expect("filed to read char") as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().expect("failed to parse token")
}

fn main() {
    let n:usize = read();
    let mut map = BTreeMap::new();
    for _ in 0..n {
        let x:usize = read();
        *map.entry(x).or_insert(0) += 1;
    }
    let mut x = 0;
    let mut y = 0;
    let mut map = map.iter();
    loop {
        if x != 0 && y != 0 {
            break;
        }
        let i = map.next_back();
        if i.is_none() {
            break;
        }

        let (k, v) = i.unwrap();
        let k = *k;
        if v >= &4 {
            x = max(x, k);
            y = max(y, k);
        } else if v >= &2 {
            if x == 0 {
                x = max(x, k);
            } else {
                y = max(y, k);
            }
        }
    }
    println!("{}", x * y);
}