Problem Statement

Three poles stand evenly spaced along a line. Their heights are $a$, $b$ and $c$ meters, from left to right. We will call the arrangement of the poles beautiful if the tops of the poles lie on the same line, that is, $b-a = c-b$.

Determine whether the arrangement of the poles is beautiful.

Constraints

• $1 \leq a,b,c \leq 100$
• $a$, $b$ and $c$ are integers.

Input

Input is given from Standard Input in the following format:

a b c

Output

Print YES if the arrangement of the poles is beautiful; print NO otherwise.