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: Accepted)

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long a = Long.parseLong(sc.next());
		//String t = sc.next();
//		String buff="";
		long b = Long.parseLong(sc.next());
		int c = Integer.parseInt(sc.next());
		//int n = Integer.parseInt(sc.next());
		//long res=n;
		//int l = Integer.parseInt(sc.next());
		//int sum=0;
		//String s = sc.next();
		//String t = sc.next();
		//String b = sc.next();
		//int a = Integer.parseInt(sc.next());
		//String t = sc.next();

		if(b-a==c-b) {//a、e、i、o、u
			System.out.println("YES");
		}else {
			System.out.println("NO");
		}
		/*for(int i=0;i<t.length();i++) {
			if(t.substring(i,i+1).equals(",")) {
				buff+=" ";
			}else {
				buff+=t.substring(i, i+1);
			}
		}*/
		//System.out.println(a*800-(a/15)*200);
	}
}