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.

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.

Constraints

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

Sample Input 1

2 4 6

Sample Output 1

YES

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

Sample Input 2

2 5 6

Sample Output 2

NO

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

Sample Input 3

3 2 1

Sample Output 3

YES

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

见题面 Input 部分

见题面 Output 部分