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)

#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define ALL(v) (v).begin(), (v).end()
#define mp make_pair
const int INF = 1e9;
const ll LINF = 1e18;
const int MOD = 1e9+7;

int main() {
	int N;
	cin >> N;
	int A[N];
	for (int i = 0; i < N; i++) cin >> A[i];

	sort(A, A+N);

	int cnt = 0, e = -1, W = 0, H = 0;

	for (int i = 0; i < N; i++) {
		if (e == A[i]) cnt++;

		if (e != A[i] || i == N-1) {
			if (cnt >= 4) {
				W = e;
				H = e;
			} else if (cnt >= 2) {
				if (e > W) {
					H = W;
					W = e;
				} else if (e > H) {
					H = e;
				}
			}
			e = A[i];
			cnt = 1;
		}
	}

	cout << (ll)W * (ll)H << endl;
    return 0;
}