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: Runtime Error)

/**
 * @author torith
 * @date 2018-10-24
 */

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

int main(){

    cin.tie(0);
    ios::sync_with_stdio(false);

    int N; cin >> N;
    int a[100000];
    for (int i = 0; i < N; ++i) {
        int t; cin >> t;
        a[t]++;
    }

    int cand1 = 0, cand2 = 0;
    for (int i = 10000; i > 0; --i) {
        if (a[i] >= 4 && cand1 == 0){
            cand1 = i;
            cand2 = i;
            break;
        }
        else if(a[i] >= 2 && cand1 == 0){
            cand1 = i;
        }else if (a[i] >= 2 && cand2 == 0){
            cand2 = i;
            break;
        }
    }

    cout << cand1 * cand2 << "\n";

    return 0;
}