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)

#define _LIBCPP_DEBUG 0
#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
ll MOD = 1e9 + 7;

int main(void){

    int N; cin>>N;
    map<ll, int> mp;
    for (int i = 0; i < N; ++i) {
        int tmp; cin>>tmp;
        mp[tmp]++;
    }

    ll fst = 0, snd = 0;
    for (auto i : mp){
        if(i.second>=2){
            if(i.first>fst){
                swap(fst, snd);
                fst = i.first;
            }else if(i.first>snd){
                snd = i.first;
            }
        }
    }
    cout<<fst*snd<<endl;

}