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: n = int(input())
  A = list(map(int, input().split()))
  up2_max = 0
  up2_2nd = 0
  up4_max = 0
  
  for cand in list(set(A)):
      num = A.count(cand)
      if num >= 2 and cand > up2_max:
          up2_max, up2_2nd = cand, up2_max
      elif num >= 4:
          up4_max = cand
  print(max(up2_max * up2_2nd, up4_max**2))
  
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"}]