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::str::FromStr;

pub fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin
        .bytes()
        .map(|c| c.expect("failed 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")
}

use std::collections::BinaryHeap;
use std::collections::HashMap;

fn main() {
    let n: usize = read();
    let mut map: HashMap<i64, i64> = HashMap::new();
    let mut a: Vec<i64> = vec![];
    for _ in 0..n {
        let ai = read();
        let mut current = map.entry(ai).or_insert(0);
        *current += 1;
    }

    let mut bh: BinaryHeap<i64> = BinaryHeap::new();

    for (k, v) in map {
        if v > 1 {
            bh.push(k);
        }
        if v > 3 {
            bh.push(k);
        }
    }

    let first = match bh.pop() {
        Some(s) => s,
        None => {
            println!("0");
            return;
        }
    };
    let second = match bh.pop() {
        Some(s) => s,
        None => {
            println!("0");
            return;
        }
    };
    println!("{}", first * second);
}