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: Wrong Answer)

#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <algorithm>
#include <map>
#include <cstdlib>
#include <cctype>
#include <string>
#include <vector>
#include <stack>
using namespace std;

#define Fill(ar, val) memset(ar, val, sizeof(ar))
#define MAX(i, j) ((i) > (j)) ? (i) : (j)
#define MIN(i, j) ((i) < (j)) ? (i) : (j)
#define ABS(i) (i) < 0 ? -(i) : i
#define PI (2 * acos(0))
#define SZ 100

int main(){

    int a, b, c;

    scanf("%d%d%d", &a, &b, &c);

    if((a < b || b > c)&& c == a){
        puts("NO");
        return 0;
    }

    if(a == b && b == c){
        puts("NO");
        return 0;
    }

    if((ABS(b - a)) == (ABS(c - b))) puts("YES");
    else puts("NO");

    return 0;

}