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:
Print the performance required to achieve the objective.
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, $2017$.
Although the current and desired ratings are between $0$ and $4500$, the performance of a user can be below $0$.