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: Wrong Answer)

#include <iostream>
#include <map>
using namespace std;
int main() {
    int N, A[100000], ans = 1, cnt = 0;
    map<int, int> m;
    map<int, int>::iterator itr;
    cin >> N;
    for (int i = 0; i < N; i++) {
        cin >> A[i];
        itr = m.find(A[i]);
        if (itr == m.end()) m[A[i]] = 1;
        else m[A[i]]++;
    }
    for (itr = m.end(); itr != m.begin(); --itr) {
        if (itr->second < 2) continue;
        ans *= itr->first;
        if (++cnt >= 2) break;
    }
    cout << (cnt != 2 ? 0 : ans) << endl;
}