output_description: Print the total horizontal length of the uncovered parts of the window.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      A B
attempt: n, k = list(map(int, input().split()))
  print(max(0, n - 2 * k))
  
problem_description: Statement
  
  The window of Takahashi's room has a width of A. There are two curtains hung
  over the window, each of which has a horizontal length of B. (Vertically, the
  curtains are long enough to cover the whole window.)
  
  We will close the window so as to minimize the total horizontal length of the
  uncovered part of the window. Find the total horizontal length of the
  uncovered parts of the window then.
samples: [{"input": "12 4", "output": "4\n    \n\nWe have a window with a horizontal length of 12, and two curtains, each of\nlength 4, that cover both ends of the window, for example. The uncovered part\nhas a horizontal length of 4.\n\n* * *"}, {"input": "20 15", "output": "0\n    \n\nIf the window is completely covered, print 0.\n\n* * *"}, {"input": "20 30", "output": "0\n    \n\nEach curtain may be longer than the window."}]