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: Wrong Answer)

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

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 mut a: Vec<usize> = s.split_whitespace().map(|x| x.parse().unwrap()).collect();
    a.sort();
    let mut last = 0;
    let mut count = 0;
    let mut x = 0;
    let mut y = 0;
    let mut iter = a.iter().rev();
    for &i in &mut iter {
        if i != last {
            if count >= 2 {
                x = last;
                count = 0;
                last = i;
                count += 1;
                break;
            }
            last = i;
            count = 0;
        }
        count += 1;
    }
    for &i in &mut iter {
        if i != last {
            if count >= 2 {
                y = last;
                // count = 0;
                // last = i;
                // count += 1;
                break;
            }
            last = i;
            count = 0;
        }
        count += 1;
    }
    if count >= 2 {
        y = last;
    }
    writeln!(output, "{}", x * y).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",
        );
    }
}