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;
int main(void){
    int N;
    cin >> N;
    vector<long long> A(N);
    for(int i = 0; i < N; i++) cin >> A[i];
    sort(A.begin(),A.end());
    reverse(A.begin(),A.end());
    long long h,w;
    h = w = 0;
    for(int i = 1; i < N; i++){
        if(A[i] == A[i-1]){
            h = A[i];
            A[i] = -1;
            A[i-1] = -2;
            break;
        } 
    }
    sort(A.begin(),A.end());
    reverse(A.begin(),A.end());
    for(int i = 1; i < N; i++){
        if(A[i] == A[i-1]){
            w = A[i];
            break;
        }
    }
    cout << h * w << endl;
}