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)

function main(input){
    var max1 = 0;
    var max2 = 0;
    var once = [];

    var tmp = input.split('\n');
    var A = tmp[1].split(' ');

    var key;
    for(var i=0; i<A.length; i++){
        key = once.index(A[i]);
        if(key===-1){
            once.push(A[i]);
        }
        else{
            once.splice(key,1);
            if(max2<A[i]){
                if(max1<A[i]){
                    max2 = max1;
                    max1 = A[i];
                }else{
                    max2 = A[i];
                }
            }
        }
    }
    console.log(max2*max1);
}

Array.prototype.index = (function (undefined)
{
    var f;

    return function (val, from)
    {
        if (from !== undefined) {
            if (from < 0) {
                f = from + this.length;

                if (f < 0)
                    return -1;
            }
            else {
                f = from;
            }
        }
        else {
            f = 0;
        }

        for (var i=f; i<this.length; i++) {
            if (this[i] === val)
                return i;
        }

        return -1;
    }
})();

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