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 C (Clang 3.8.0) (judge verdict: Accepted)

#include<stdio.h>

int main(){
	int a,b,c,jd1,jd2;
	
	scanf("%d %d %d", &a, &b, &c);

	if(a<1 || a>101 || b<1 || b>101 || c<1 || c>101) {
		exit(0);
	}

	jd1 = b - a; jd2 = c - b;

	if(jd1 == jd2){
		puts("YES");
	} else {
		puts("NO");
	}
	return 0;
}