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 Java8 (OpenJDK 1.8.0) (judge verdict: Runtime Error)

import java.io.InputStream;
import java.io.PrintStream;
import java.util.Scanner;

public class Main{
	static Scanner sc;

	public static void main(String[] args) throws Exception {
		//InputStream in = new FileInputStream("src/input.txt");
		InputStream in = System.in;
		PrintStream out = new PrintStream("src/output.txt");

		//System.setOut(out);

		sc = new Scanner(in);

		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = sc.nextInt();
		if(b - a == c - b){
			System.out.println("OK");
		}else{
			System.out.println("NG");
		}

		out.close();
	}
}