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)

#![allow(non_snake_case)]
#![allow(unused_variables)]
#![allow(dead_code)]

fn main() {
    let N: usize = {
        let mut line: String = String::new();
        std::io::stdin().read_line(&mut line).unwrap();
        line.trim().parse().unwrap()
    };
    let A: Vec<i64> = {
        let mut line: String = String::new();
        std::io::stdin().read_line(&mut line).unwrap();
        line.split_whitespace()
            .map(|x| x.parse().unwrap())
            .collect()
    };

    let mut c = std::collections::HashMap::new();
    for &a in &A { *(c.entry(a).or_insert(0)) += 1; }
    let d = {
        let mut d = c.iter().map(|(&k, &v)| (k, v)).collect::<Vec<_>>();
        d.sort_by_key(|&(k, v)| -k);
        d
    };
    let (ans, _) = d.iter().fold((0, 0), |(res, m), &(k, v)|
        if m == 0 && v >= 4 { (k * k, 2) }
        else if m == 1 && v >= 2 { (res * k, m + 1) }
        else if m == 0 && v >= 2 { (k, m + 1) }
        else { (res, m) }
    );

    println!("{}", ans);
}