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 <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <cassert>
#include <iostream>
#include <sstream>
#include <fstream>
#include <string>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <utility>
#include <stack>
#include <numeric>
#include <algorithm>
#include <bitset>
#include <complex>
using namespace std;
typedef long long ll;
typedef long long int llint;
typedef pair<ll, ll> pa;
#define MM 1000000000
#define MOD MM+7
#define MAX 101000
#define MAP 110
#define initial_value -1
#define MAX_T 1001
#define Pair pair<int,int>
#define chmax(a,b) (a<b ? a=b:0)
#define chmin(a,b) (a>b ? a=b:0)
#define INF (1 << 29) //536870912
const long double PI = acos(-1);
const ll DEP = 1e18;
int dx[4] = {-1,0,1,0};
int dy[4] = {0,-1,0,1};
int main(){
    int a,b,c; cin >> a >> b >> c;
    if(b-a == c-b){
        cout << "YES" << endl;
    } else {
        cout << "NO" << endl;
    }
}