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>
#define rep(i, n) for(ll i = 0; i < (n); ++i)
using namespace std;
using ll = long long;
const double PI=acos(-1);

int main(){
    ll n;
    cin >> n;
    map<ll, ll> l;
    rep(i, n){
        ll o;
        cin >> o;
        l[o]++;
    }
    
    ll a = 0, b = 0;
    for(auto i = l.rbegin(); i != l.rend(); ++i){
        printf("%lld %lld\n", i->first, i->second);
        if(i->second/2 >= 1){
            if(i->second/2 >= 2){
                if(a == 0 && b == 0) a = b = i->first;
                else b = i->first;
            }else{
                if(a == 0) a = i->first;
                else b = i->first;
            }
            if(a != 0 && b != 0) break;
        }
    }
    
    ll ans  = a*b;
    cout << ans << endl;
    return 0;
}