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 9.2.1) (judge verdict: Accepted)

//xtwakioi! xtwddYnoi(双重含义)!
#include <bits/stdc++.h>
#define ri register
#define int long long
#define E (n+1)
using namespace std;
inline int read()
{
    int s=0, w=1; ri char ch=getchar();
    while(ch<'0'||ch>'9') { if(ch=='-') w=-1; ch=getchar(); }
    while(ch>='0'&&ch<='9') s=(s<<3)+(s<<1)+(ch^48), ch=getchar();
    return s*w;
}
signed main()
{
    int a,b,c;
    cin>>a>>b>>c;
    if(b*2==a+c) puts("YES");
    else puts("NO");
    return 0;
}