Score : 100 points
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:
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.
Input is given from Standard Input in the following format:
R G
Print the performance required to achieve the objective.
2002 2017
2032
Takahashi's current rating is 2002.
If his performance in the contest is 2032, his rating will be the average of 2002 and 2032, which is equal to the desired rating, 2017.
4500 0
-4500
Although the current and desired ratings are between 0 and 4500, the performance of a user can be below 0.