#define	_USE_MATH_DEFINES
#include <iostream>
#include <iomanip>
#include <cctype>
#include <cstdlib>
#include <algorithm>
#include <functional>
#include <vector>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <cfloat>
#include <map>
#include <queue>
#include <stack>
#include <list>
#include <string>
#include <set>
#include <complex>
#include <utility>
#include <numeric>
#define  rep(i,n) for(int i=0;i<(n);i++)
#define  REP(i,a,n) for(int i=a;i<(n);i++)
#define  rrep(i,n) for(int i=(n)-1;i>=0;i--)
#define  VI	vector<int>
#define	 VS vector<string>
#define  all(a) (a).begin(),(a).end()
#define  debug(x) cout<<#x<<": "<<x<<endl

using namespace std;
typedef long long ll;
typedef string::const_iterator State;
typedef pair<int,int> P;
class ParseError {};
const int INF=1e9;
char fi[101][101];
int day[12]={31,28,31,30,31,30,31,31,30,31,30,31};
double EPS = 1e-14;
const int MAX_V=100;
const int MAX_N=100;
char o[3]={'+','-','*'};
#define md 1000003



int bow[353][353]={0};
double add(double a,double b){
	if(abs(a+b)<EPS*(abs(a)+abs(b)))
	return 0;
	return a+b;
}

/*struct P{
	double x,y;
	P(){}
		P(double x,double y):x(x),y(y){
		}
		P operator + (P p){
			return P(add(x,p.x),add(y,p.y));
		}
		P operator - (P p){
			return P(add(x,-p.x),add(y,-p.y));
		}
		P operator *(double d){
			return P(x*d,y*d);
		}
		double dot(P p){
			return add(x*p.x,y*p.y);
		}
		double det(P p){
			return add(x*p.y,-y*p.x);
		}
};

bool cmp_x(const P& p,const P& q){
	if(p.x!=q.x) return p.x<q.x;
	return p.y<q.y;
}

vector<P> convex_hull(P* ps, int n){
	sort(ps,ps+n,cmp_x);
	int k=0;
	vector<P> 	qs(n*2);
	
	rep(i,n){
		while(k>1&&(qs[k-1]-qs[k-2]).det(ps[i]-qs[k-1])<=0)
			k--;
		qs[k++]=ps[i];
	}
	for(int i=n-2,t=k;i>=0;i--){
		while(k>t&&(qs[k-1]-qs[k-2]).det(ps[i]-qs[k-1])<=0)
		k--;
		qs[k++]=ps[i];
	}
	qs.resize(k-1);
	return qs;
}

int n,m;
vector<double> p;
P ps[101];
*/
char c[520][520];
long long mod=1000000007;
long long pow(ll i,ll j){
	ll tmp=1;
	while(j){
		if(j%2) tmp=tmp*i%mod;
		i=i*i%mod;
		j/=2;
	}
	return tmp;
}

int expression(State&);
int term(State&);
int factor(State&);
int number(State&);
int expression(State &begin){
	int ret = term(begin);
	
	for(;;){
		if(*begin == '+'){
			begin++;
			ret += term(begin);
		}
		else if(*begin == '-'){
			begin++;
			ret -= term(begin);
		}
		else
			break;
	}
	return ret;
}

int term(State &begin){
	int ret = factor(begin);
	
	for(;;){
		if(*begin=='*'){
			begin++;
			ret *= factor(begin);
		}
		else if(*begin=='/'){
			begin++;
			ret /= factor(begin);
		}
		else
			break;
	}
	return ret;
}

int factor(State &begin){
	int ret;
	if(*begin == '('){
		begin++;
		ret = expression(begin);
		begin++;
	}
	else
	ret = number(begin);
	
	return ret;

}

int number(State &begin){
	int ret =0;
	
	while(isdigit(*begin)){
		ret*=10;
		ret+=*begin - '0';
		begin++;
	}
	return ret;
}

	
		
int henkan(string s){
	
	map<char, int> moji;
	moji['m']=1000;
	moji['c']=100;
	moji['x']=10;
	moji['i']=1;
	int value=0;
	int temp=1;
	for(int i=0;i<s.size();i++){
		if(s[i]>='2'&&s[i]<='9')
			temp=temp*(s[i]-'0');
		else{
			value+=temp*moji[s[i]];
			temp=1;
		}
	}
	
	return value;
}


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


Measured memory_bytes: 256