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A - ι⊥l

Score : $100$ points

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.

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.