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;

fn gets() -> String {
    let mut buf = String::new();
    io::stdin().read_line(&mut buf).ok();
    return buf
}

fn main() {
  let line = gets();
  let n = line.trim().parse::<usize>().unwrap();
  let line = gets();
  let mut a: Vec<i64> = line.split_whitespace().map(|x| x.parse::<i64>().unwrap()).collect();
  let mut l = &0;
  let mut s = &0;
  a.sort_by(|x, y| y.cmp(x));
  let mut it = a.iter();
  let mut curr = it.next().unwrap();
  let mut cnt = 1;
  loop {
    let x = it.next();
    match x {
      Some(i) if i == curr => cnt = cnt + 1, 
      Some(i) => {
        if cnt >= 4 {
          if l == &0 {
            l = curr;
          }
          if s == &0 {
            s = curr;
          }
        } else if cnt >= 2 {
          if l == &0 {
            l = curr;
          } else if s == &0 {
            s = curr;
          }
        }
        curr = i;
        cnt = 1;
      }, 
      None => {
        if cnt >= 4 {
          if l == &0 {
            l = curr;
          }
          if s == &0 {
            s = curr;
          }
        } else if cnt >= 2 {
          if l == &0 {
            l = curr;
          } else if s == &0 {
            s = curr;
          }
        }
        println!("{}", l * s);
        break;
      }, 
    }
  }
}