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)

// Created by sz
#include <bits/stdc++.h>
using namespace std;

const int N = 1e5 + 5;
int n;
map<long long, int> cnt;

int main(){
#ifdef LOCAL
    freopen("./input.txt", "r", stdin);
#endif
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    
    cin>>n;
    long long x;
    for (int i = 1; i <= n; i++){
        cin>>x;
        cnt[x] ++;
    }
    
    vector<int> sid;
    for (auto i = cnt.rbegin(); i != cnt.rend(); i++){
        if(i->second >=2)sid.push_back(i->first);
        if(sid.size()==2)break;
    }
    
    if (sid.size()<2)cout<<0<<endl;
    else cout<<sid[0]*sid[1]<<endl;
    
    
    return 0;
}