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 <bits/stdc++.h>
using namespace std;

int main() {
    int N;
    cin >> N;
    vector<int> A(N);
    map<int,int> C;
    for(int i=0; i<N; ++i) {
        cin >> A[i];
        C[A[i]] += 1;
    }

    vector<int> B;
    for(auto itr = C.begin(); itr != C.end(); ++itr) {
        if(itr->second >= 2) {
            B.push_back(itr->first);
        }
        if(itr->second >= 4) B.push_back(itr->first);
    }
    sort(B.begin(), B.end(), greater<int>());
    if(B.size() < 2) cout << 0 << endl;
    else cout << B[0] * B[1] << endl;
}