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 <iostream>
#include <array>
#include <algorithm>

using namespace std;

using ll = long long int;

int main(){
    
    ll N;
    array<ll, 200000> A;
    
    cin >> N;
    for(ll i = 0; i < N; ++i){
        cin >> A[i];
    }
    
    sort(A.begin(), A.begin() + N, [](auto a, auto b)->bool{
        return a > b;
    });
    
    ll last = -1;
    ll lastcnt = 1;
    ll cnt = 0;
    ll ans[2] = {0};
    for(ll i = 0; i < N; ++i){
        if(A[i] == last){
            lastcnt += 1;
            if(lastcnt == 2){
                ans[cnt++] = last;
                if(cnt >= 2)break;
                lastcnt = 0;
            }
        }else{
            last = A[i];
            lastcnt = 1;
        }
    }
    
    cout << ans[0] * ans[1] << endl;
    
    return 0;
}