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: Wrong Answer)

#include <stdio.h>

int main()
{
	int a, b, c;
	scanf( "%d %d %d", &a, &b, &c );
	
	int is_up;
	int diff_ab;
	if ( b > a )
	{
		is_up = 1;
		diff_ab = b - a;
	}
	else
	{
		is_up = 0;
		diff_ab = a - b;
	}
	
	int diff_cb;
	if ( is_up )
	{
		diff_cb = c - b;
	}
	else
	{
		diff_cb = b - c;
	}
	
	
	if ( diff_ab == diff_cb )
	{
		printf( "Yes\n" );
	}
	else
	{
		printf( "No\n" );
	}
	
	return 0;
}