output_description: Print the performance required to achieve the objective.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      R
      G
attempt: g,r=map(int,input())
  print(2r-g)
problem_description: Statement
  
  Takahashi is a user of a site that hosts programming contests.  
  When a user competes in a contest, the _rating_ of the user (not necessarily
  an integer) changes according to the _performance_ of the user, as follows:
  
    * Let the current rating of the user be a.
    * Suppose that the performance of the user in the contest is b.
    * Then, the new rating of the user will be the avarage of a and b.
  
  For example, if a user with rating 1 competes in a contest and gives
  performance 1000, his/her new rating will be 500.5, the average of 1 and 1000.
  
  Takahashi's current rating is R, and he wants his rating to be exactly G after
  the next contest.  
  Find the performance required to achieve it.
samples: [{"input": "2002\n    2017", "output": "2032\n    \n\nTakahashi's current rating is 2002.  \nIf his performance in the contest is 2032, his rating will be the average of\n2002 and 2032, which is equal to the desired rating, 2017.\n\n* * *"}, {"input": "4500\n    0", "output": "-4500\n    \n\nAlthough the current and desired ratings are between 0 and 4500, the\nperformance of a user can be below 0."}]