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 v5.12) (judge verdict: Wrong Answer)

"use strict"

const main = arg => {
  arg = arg.trim().split("\n");
  const N = parseInt(arg[0]);
  const A = arg[1].split(" ").map(n=>parseInt(n)).sort((a,b)=>a-b);
  
  let pairNums = [];
  
  for(let i in A) {
      if(A[i] === A[parseInt(i) + 1]) {
        pairNums.push(A[i]);
      }
  }
  
  pairNums.sort((a,b)=>b-a);
  const set = new Set(pairNums);
 
  if([...set].length === 1) {
      console.log([...set][0] * [...set][0]);
      return;
  }

  console.log(pairNums.length ? [...set][0] * [...set][1] : 0);
}

main(require("fs").readFileSync("/dev/stdin", "utf8"));