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)

//#define _GLIBCXX_DEBUG

#include<bits/stdc++.h>
#define PI 3.14159265359
using namespace std;
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
const int INF= 1e9+5;
typedef long long ll;
typedef vector<ll> vl;
typedef vector<vector<ll> >vvl;
typedef pair<ll,ll> P;
typedef tuple<ll,ll,ll> T;
const ll MOD=1000000007LL;
int main(){
  ll n;cin>>n;
  vl a(n);
  rep(i,n)cin>>a[i];
  vector<ll>ans;
  map<ll,ll>M;
  rep(i,n)M[a[i]]++;
  for(auto p:M){
    if(p.second>=4){
      ans.push_back(p.first);
      ans.push_back(p.first);
    }
    else if(p.second>=2){
      ans.push_back(p.first);
    }
  }
  sort(ans.begin(),ans.end());
  reverse(ans.begin(),ans.end());
  if(ans.size()<2){
    cout<<0<<endl;
    return 0;
  }
  cout<<ans[0]*ans[1]<<endl;
  return 0;
}