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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <iostream>
#include <map>
using namespace std;
long long n, sti, a, b;
map<int, int> mp;
int main(void){
    // Your code here!
    cin >> n;
    for (int i = 0; i < n; i++) {
        cin >> sti;
        if (mp[sti] == 1) {
            if (sti > a) b = a, a = sti;
            else if (sti > b) b = sti;
        } if (mp[sti] == 3) {
            if (sti == a) b = sti;
        } 
        mp[sti]++;
    }
    cout << a * b << endl;
}