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:  = int(input())
  a = list(map(int, input().split()))
  
  bar_list = {}
  for i in a:
  	if i in bar_list:
  		bar_list[i] += 1
  	else:
  		bar_list[i] = 1
  
  saved = -1
  ans = 0
  for k, v in sorted(bar_list.items(), key=lambda x:x[1]):
  	if v > 1:
  		if saved < 0:
  			saved = k
  		else:
  			ans = saved * k
  			break
  print(ans)
  
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"}]