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 proconio::input;
// ordtools {{{
#[allow(dead_code)]
mod ordtools {
    pub trait Ordtools: PartialOrd + Sized {
        fn change_min(&mut self, mut rhs: Self) {
            if self > &mut rhs {
                *self = rhs;
            }
        }

        fn change_max(&mut self, mut rhs: Self) {
            if self < &mut rhs {
                *self = rhs;
            }
        }
    }

    impl<T: PartialOrd + Sized> Ordtools for T {}
}
// }}}
use ordtools::Ordtools;
fn main() {
    input!(n: usize, a: [u64; n]);
    let mut map = std::collections::BTreeMap::new();
    for x in a {
        *map.entry(x).or_insert(0) += 1;
    }
    let seq = map.iter().filter(|&(_, &cnt)| 2 <= cnt).collect::<Vec<_>>();
    let mut ans = 0;
    if let Some((&x, _)) = seq.iter().rfind(|&(_, &cnt)| 4 <= cnt) {
        ans.change_max(x * x);
    }
    if 2 <= seq.len() {
        ans.change_max(seq[seq.len() - 2].0 * seq[seq.len() - 1].0);
    }
    println!("{}", ans);
}