output_description: Print the maximum possible area of the rectangle. If no rectangle can be
  formed, print 0.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
      A_1 A_2 ... A_N
attempt: #include <iostream>
  #include <cmath>
  #include <vector>
  #include <string>
  using namespace std;
  
  typedef long long ll;
  typedef unsigned long long ull;
  struct edge { int u, v; ll w; };
   
  ll MOD = 1000000007;
  ll _MOD = 1000000009;
  double EPS = 1e-10;
  
  int main() {
    int N;
    int A[100000];
    cin >> N;
    for(int i = 0; i < N; i++)
      cin >> A[i];
  
  }
  
problem_description: 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.
samples: [{"input": "6\n    3 1 2 4 2 1", "output": "2\n    \n\n1 \\times 2 rectangle can be formed.\n\n* * *"}, {"input": "4\n    1 2 3 4", "output": "0\n    \n\nNo rectangle can be formed.\n\n* * *"}, {"input": "10\n    3 3 3 3 4 4 4 5 5 5", "output": "20"}]