output_description: Print the minimum total height of the stools needed to meet the goal.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N
      A_1 \ldots A_N
attempt: n = int(input())
  a = map(int, input().split())
  a = list(set(a)).sort()
  max = 0
  for i in range(len(a) - 1):
      if a[i] - a[i - 1] > max:
          max = a[i] = a[i - 1]
  print(max)
  
problem_description: Statement
  
  N persons are standing in a row. The height of the i-th person from the front
  is A_i.
  
  We want to have each person stand on a stool of some heights - at least zero -
  so that the following condition is satisfied for every person:
  
  Condition: Nobody in front of the person is taller than the person. Here, the
  height of a person includes the stool.
  
  Find the minimum total height of the stools needed to meet this goal.
samples: [{"input": "5\n    2 1 5 4 3", "output": "4\n    \n\nIf the persons stand on stools of heights 0, 1, 0, 1, and 2, respectively,\ntheir heights will be 2, 2, 5, 5, and 5, satisfying the condition.\n\nWe cannot meet the goal with a smaller total height of the stools.\n\n* * *"}, {"input": "5\n    3 3 3 3 3", "output": "0\n    \n\nGiving a stool of height 0 to everyone will work."}]