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>
using namespace std;

int main(void) {
unsigned long int n, m;
signed long int x[100000], y[100000];
unsigned long int a,b,c;
c = 0;
cin >> n >> m;
for (int i = 0; i < n; i++) {
cin >> x[i];
}
for (int i = 0; i < m; i++) {
cin >> y[i];
}
for (int i = 1; i < n; i++) {
for (int j = 1; j < m; j++) {
for (int k = 0; k < n-i; k++) {
a = x[k+i] - x[k];
for (int l = 0; l < m-j; l++) {
b = y[l+j] - y[l];
c = c + a*b;
c = c % 1000000007;
}
}
}
}
cout << c <<endl;
}