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>

 int main(void){
      int a,b,c;
      char dummy;
      int d,e,f;
      
      /*printf("柱の高さを３つ決めてください\n");
      printf("左　中　右\n");*/
      rewind(stdin);
      d = scanf("%d %d %d",&a,&b,&c);
     /* printf("柱の高さを３つ決めてください\n");
      printf("左 ");
      rewind(stdin);
      d = scanf("%d",&a); 
      printf("中 ");
      rewind(stdin);
      e = scanf("%d",&b); 
      printf("右 ");
      rewind(stdin);
      f = scanf("%d",&c); */
       
      if(a<1 || a>100 || b<1 || b>100 || c<1 || c>100){
        printf("柱は　１〜１００　の間で決めてください\n");
        exit(0);
        }
      
      if(b-a == c-b){
        printf("YES\n");
        return 0;
      }else{
        printf("NO\n");
        return 0;
       }
   }