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;
use std::collections::HashMap;

fn main() {
  input! {
    n: usize,
    a: [i64; n],
  };
  
  let mut counts: HashMap<i64, i64> = HashMap::new();
  
  for x in a {
    if let Some(&count) = counts.get(&x) {
      counts.insert(x, count+1);
    } else {
      counts.insert(x, 1);
    }
  }
  
  let mut pairs: Vec<i64> = Vec::new();
  
  for (&x, &count) in &counts {
    for _ in 0..(count/2) {
      pairs.push(x);
    }
  }
  
  // 降順に並び替え
  pairs.sort_by_key(|&x| -x);
  
  if pairs.len() < 2 {
    println!("0");
  } else {
    println!("{}", pairs[0] * pairs[1]);
  }
}