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)

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

fn main() {
  let line1 = getline();
  let _n:i32 = line1.trim().parse().unwrap();
  let line2 = getline();
  let mut vs:Vec<i64> = line2.trim().split(' ').map(|v| v.parse().unwrap()).collect();
  vs.sort();
  vs.reverse();

  let mut candidate = Vec::new();
  let mut prev = -1;
  for v in vs {
    if v == prev {
      candidate.push(v);
      prev = -1;
    } else {
      prev = v;
    }
  }
  candidate.push(0);
  candidate.push(0);
  let ans = candidate[0] * candidate[1];
  println!("{}", ans);
}