output_description: Print the maximum number of days Takahashi can hang out during the vacation,
  or `-1`.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N M
      A_1 ... A_M
attempt: N, M = [int(el) for el in input().split()]
  sm = sum([int(el) for el in input().split()])
  print(-1 if N < sm else N - sm)
  
problem_description: Statement
  
  Takahashi has N days of summer vacation.
  
  His teacher gave him M summer assignments. It will take A_i days for him to do
  the i-th assignment.
  
  He cannot do multiple assignments on the same day, or hang out on a day he
  does an assignment.
  
  What is the maximum number of days Takahashi can hang out during the vacation
  if he finishes all the assignments during this vacation?
  
  If Takahashi cannot finish all the assignments during the vacation, print `-1`
  instead.
samples: [{"input": "41 2\n    5 6", "output": "30\n    \n\nFor example, he can do the first assignment on the first 5 days, hang out on\nthe next 30 days, and do the second assignment on the last 6 days of the\nvacation. In this way, he can safely spend 30 days hanging out.\n\n* * *"}, {"input": "10 2\n    5 6", "output": "-1\n    \n\nHe cannot finish his assignments.\n\n* * *"}, {"input": "11 2\n    5 6", "output": "0\n    \n\nHe can finish his assignments, but he will have no time to hang out.\n\n* * *"}, {"input": "314 15\n    9 26 5 35 8 9 79 3 23 8 46 2 6 43 3", "output": "9"}]