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 (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
#define rep(i,n) for(i=0;i<n;i++)

int in(void){
	int i;
	scanf("%d",&i);
	return i;
}
long long llin(void){
	long long i;
	scanf("%lld",&i);
	return i;
}
int cmp(const void *a,const void *b){
	return *(long long *)a-*(long long *)b;
}
long long min(long long a,long long b){
	return a<b?a:b;
}
long long max(long long a,long long b){
	return a>b?a:b;
}

int main(void){
	int a,b,c;
	a=in();
	b=in();
	c=in();
	puts(b-a==c-b?"YES":"NO");
	return 0;
}