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++ (Clang 10.0.0) (judge verdict: Runtime Error)

#include <iostream>
using namespace std;

int main() {
	int n;
	cin>>n;
	long long arr[n];
	for (int i=0; i<n; i++){
		cin >> arr[i];
	}
	long long b[1000000001]={};
	for (int i=0; i<n; i++){
		b[arr[i]]++;
	}
	long long area=1;
	long long x=1;
	int cnt=0;
	for (long long i=1000000000; i>0; i--){
		if (b[i]>1){
			x *= i;
			//cout << i<< " ";
			cnt++;
			if (cnt==2){
				cout << x << endl;
				return 0;
			}
		}
	}
	cout << 0 << endl;
	
}