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)

/*
  第4回ユニリタプログラミングコンテスト～練習サイト～
  C言語用
  ファイル名 qA.c
  作成者 228 金子幸裕 2017/06/14 12:50
*/
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>

int main( void )
{
    int na, nb, nc;

    scanf( "%d %d %d" , &na, &nb, &nc );

    if ( (nb - na) == (nc - nb) )
    {
        printf("YES");
    }
    else
    {
        printf("NO");
    }


    printf("\n");

    return ( 0 );

}