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, fastout};
use proconio::marker::{Chars};
use proconio::marker::Usize1;
use std::cmp::*;

macro_rules! debug {
      ($($a:expr),*) => {
          eprintln!(concat!($(stringify!($a), " = {:?}, "),*), $($a),*);
    }
}

#[fastout]
fn main() {
    input!{
        n: usize,
        mut a: [usize;n],
    }
    a.sort();
    a.push(0);
    let mut b: Vec<(usize,usize)> = Vec::new();
    let mut cnt = 0;
    for i in 0..n {
        let tmp = a[i];
        cnt += 1;
        if a[i+1]==tmp {
            continue;
        } else {
            debug!(cnt);
            if cnt>=2 {
                b.push((tmp, cnt));
            }
            cnt = 0;
        }
    }
    debug!(b);
    if b.len()==0 {
        println!("0");
    } else if b.len()==1 {
        if b[0].1<4 {
            println!("0");
        } else {
            println!("{}", b[0].0*b[0].0);
        }
    } else {
        if b[b.len()-1].1<4 {
            println!("{}", b[b.len()-1].0*b[b.len()-2].0);
        } else {
            println!("{}", b[b.len()-1].0*b[b.len()-1].0);
        }
    }

}