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<bits/stdc++.h>
typedef long long ll;
using namespace std;

int main(){
    int n; cin >> n;
    vector<ll> lis(n);
    for(int i = 0; i < n; i++){
        cin >> lis[i];
    }
    sort(lis.begin(), lis.end(), greater<>());
    ll max1 = 0, max2 = 0;
    for(int i = 0; i < n-1; i++){
        if(max1 == 0 && lis[i] == lis[i+1]){
            max1 = lis[i];
            i += 2;
        }
        if(i > n-1){break;}
        if(max2 == 0 && lis[i] == lis[i+1]){
            max2 = lis[i];
        }
    }
    cout << max1 * max2 << endl;
}