Contest: Task: Related: TaskB

Score : $100$ points

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.

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

Input is given from Standard Input in the following format:

$a$ $b$ $c$

Print `YES`

if the arrangement of the poles is beautiful; print `NO`

otherwise.

2 4 6

YES

Since $4-2 = 6-4$, this arrangement of poles is beautiful.

2 5 6

NO

Since $5-2 \neq 6-5$, this arrangement of poles is not beautiful.

3 2 1

YES

Since $1-2 = 2-3$, this arrangement of poles is beautiful.