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.

-----Sample Input-----
2 4 6

-----Sample Output-----
YES

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

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