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>
#define INF (1<<30)
#define MOD 1000000007
#define l_ength size
using ll = long long;
using namespace std;

int main(){
	int n; cin >> n;
	vector<ll> a(n);
	for( auto &k : a ) cin >> k;
	sort( a.begin(), a.end(), greater<ll>() );
	vector<ll> candy;
	int cnt = 0;
	for( int i = 1; i < n; ++i ){
		if( cnt == 2 ) break;
		if( a[i-1] == a[i] ){
			candy.push_back(a[i]);
			++cnt;
			++i;
		}
	}
	if( cnt == 2 ){
		cout << candy[0]*candy[1] << endl;
		return 0;
	} else {
		cout << 0 << endl;
		return 0;
	}		
	return 0;
}