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>
using namespace std;
typedef long long ll;
#define pb push_back
#define all(v) v.begin(),v.end()
#define PI acos(-1)
#define EPS 1e-15
#define f first
#define s second
 
const int N = 1e5 + 15;
 
int cnt[N];
 
set<int> s;
 
int main(){
	int n;
	cin >> n;
	map<ll, ll> m;
	for(int i=0; i < n; i++){
		int x;
		cin >> x;
		m[x]++;
	}
	vector<ll> v;
	ll res = 0;
	for(auto it = m.begin(); it != m.end(); it++){
		if(it->second < 2)
			continue;
		if(it->second >= 4)
			res = max(res, (it->first) * (it->first));
			
		v.pb(it->first);
		
	}
	
	if(v.size() > 0){
		int l = v.size();
		res = max(res, v[l-1] * v[l-2]);
	}
	cout << res << endl;
	return 0;
}