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<string.h>
int a[4];
void abcd(int l,int r)
{
    int i=l,j=r,t;
    int p=a[(l+r)/2];
    while(i<=j)
    {while(a[i]<p)
    {i++;}
    while(a[j]>p)
    {j--;}
    if(i<=j)
    {t=a[i]; 
    a[i]=a[j]; 
    a[j]=t;
    i++; 
    j--;}}
    if(l<j)
    abcd(l,j);
    if(i<r)
    abcd(i,r); 
    }
int main()
{
    scanf("%d%d%d",&a[1],&a[2],&a[3]);
    abcd(1,3);
    if(a[2]-a[1]==a[3]-a[2])
    printf("YES\n");
    else
    printf("NO\n");
    return 0;
}