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)

// Header {{{
#include <bits/stdc++.h>

using namespace std;
template<typename T>
using reversed_priority_queue = std::priority_queue<T, std::vector<T>, std::greater<T> >;
typedef unsigned long long int ull;
typedef long long int ll;
#define rep(i,a,b) for (ll i=(a); i<(b); i++)
typedef pair<ll,ll> PLL;
// }}}

signed main() {
    ll n;
    cin>>n;
    map<ll,ll> m; // value -> num of value
    rep(i,0,n){
        ll a;
        cin>>a;
        m[a]++;
    }
    vector<ll> v;
    for(auto &e : m)
        if(e.second>=4){
            v.push_back(e.first);
            v.push_back(e.first);
        }else if(e.second>=2){
            v.push_back(e.first);
        }
    sort(begin(v),end(v),greater<ll>());
    if(v.size()<2)
        cout<<0<<endl;
    else
        cout<<v[0]*v[1]<<endl;
    return 0;
}