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 JavaScript (Node.js 12.16.1) (judge verdict: Accepted)

console.log((args=>{
  const [[N],A] = args.trim().split('\n').map(r=>r.split(' ').map(v=>v|0));
  const a = new Map();
  for ( let i = 0; i < N; i++ ) {
    a.set(A[i],(a.get(A[i])|0)+1);
  }
  const e = [];
  for ( const [k,v] of a ) {
    if ( v >= 2 ) e.push(k);
    if ( v >= 4 ) e.push(k);
  }
  if ( e.length < 2 ) return '0';
  e.sort((a,b)=>b-a);
  return `${BigInt(e[0])*BigInt(e[1])}`;
})(require('fs').readFileSync('/dev/stdin', 'utf8')));