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 JavaScript (node.js v5.12) (judge verdict: Accepted)

function Main( input ){
 
    input = input.split( ' ' )
 
    var a = Number( input[0] )
    var b = Number( input[1] )
    var c = Number( input[2] )
 
    if( b - a === c - b ){
        console.log( 'YES' )
    } else {
        console.log( 'NO' )
    }
 
}
 
Main( require( 'fs' ).readFileSync( '/dev/stdin', 'utf8' ) )