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)

# include <bits/stdc++.h>
typedef long long lint;
using namespace std;

lint gcd(lint x, lint y) {
  if(x == 0) { return y; }
  else { return gcd(y%x,x); }
}

lint lcm(lint x, lint y) { return x/gcd(x,y)*y; }

lint C(lint n, lint k) {
  if(n == k) { return 1; }
  else if(n < k) { return 0; }
  else if(k == 0) { return 1; }
  else if(k == 1) { return n; }
  else return C(n-1,k-1) + C(n-1,k);
}


int main() {
  lint n; cin >> n;
  vector<lint> a(n); for(lint i=0;i<n;i++) { cin >> a[i]; }

  sort(a.begin(),a.end(),greater<lint>());

  vector<lint> lmax(2,0);
  for(lint i=1;i<n;i++) {
    if(a[i] == a[i-1]) {
      lmax.push_back(a[i]);
      i++;
    }
    if((int)lmax.size() >= 4) { break; }
  }

  sort(lmax.begin(),lmax.end(),greater<lint>());
  cout << lmax[0] * lmax[1] << endl;
  return 0;
}