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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
#define gcd(m,n) __gcd(m,n)
#define lcm(m,n) m*(n/gcd(m,n))
#define fast std::ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define pi acos(-1.0)
#define endl '\n'
#define MOD 1000000007
#define ull unsigned long long
#define ll long long
#define ld long double
#define pb push_back
#define dbg(x) cout << #x << "  " << x << endl;

ll power(ll base, ll exp) 
{ll res=1;while(exp>0) {if(exp%2==1) res=(res*base);base=(base*base);exp/=2;}return res;}
ll mod(ll a, ll b) {return (a % b + b) % b;}
using namespace std;

int main() 
{
	fast;
	ll a,b,c; cin>>a>>b>>c; if(b-a == c-b) cout << "YES" << endl; else cout << "NO" << endl;
  	return 0; 
}