Problem:
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.

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.

Solution in Scala (2.11.7) (judge verdict: Accepted)

object Main extends App {
	def minus(x: Int, y: Int): Int = {
		return y - x;
	}

	val N = io.StdIn.readLine;

	val a = N.split(" ")(0).toInt;
	val b = N.split(" ")(1).toInt;
	val c = N.split(" ")(2).toInt;

	if(minus(a,b) == minus(b,c)) println("YES")
	else println("NO")
}