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::{BufRead, Write};
use std::io;
use std::collections::BTreeMap;

fn solve<R: BufRead, W: Write>(mut input: R, mut output: W) {
    let mut s = String::new();
    input.read_line(&mut s).unwrap();
    let _: usize = s.trim().parse().unwrap();
    s.clear();
    input.read_line(&mut s).unwrap();
    let a: Vec<usize> = s.split_whitespace().map(|x| x.parse().unwrap()).collect();
    let mut map: BTreeMap<usize, usize> = BTreeMap::new();
    for x in a {
        *map.entry(x).or_insert(0) += 1;
    }
    let mut it = map.iter().rev().flat_map(|(a, b)| if *b >= 4 {
        vec![*a, *a]
    } else if *b >= 2 {
        vec![*a]
    } else {
        vec![]
    });
    writeln!(
        output,
        "{}",
        it.next().unwrap_or(0) * it.next().unwrap_or(0)
    ).unwrap();
}

fn main() {
    let stdin = io::stdin();
    let stdout = io::stdout();
    solve(stdin.lock(), stdout.lock());
}

#[cfg(test)]
mod tests {
    fn test(input: &str, output: &str) {
        let mut v: Vec<u8> = Vec::new();
        ::solve(input.as_bytes(), &mut v);
        assert_eq!(String::from_utf8(v).unwrap(), output);
    }

    #[test]
    fn case1() {
        test(
            "6
3 1 2 4 2 1\n",
            "2\n",
        );
    }
    #[test]
    fn case2() {
        test(
            "4
1 2 3 4\n",
            "0\n",
        );
    }
    #[test]
    fn case3() {
        test(
            "10
3 3 3 3 4 4 4 5 5 5\n",
            "20\n",
        );
    }
    #[test]
    fn case4() {
        test(
            "4
5 5 5 5\n",
            "25\n",
        );
    }
}