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 (GCC 5.4.1) (judge verdict: Wrong Answer)


#include <stdio.h>
#include <string.h>

#define STRING_LEN 50

void GetsInput(char *str)
{
    if (fgets(str, STRING_LEN, stdin))
    {
        char *find = strchr(str, '\n');
        if (find)
        {
            *find = '\0';
        }
        else
        {
            while (getchar() != '\n')
                continue;
        }
    }
}

int main(void)
{
    char O[STRING_LEN + 1];
    char E[STRING_LEN + 1];
    char R[STRING_LEN * 2 + 1];

    GetsInput(O);
    GetsInput(E);

    for (int i = 0; O[i] != '\0'; i++)
    {
       int result_index = i * 2;
       R[result_index] = O[i];
       R[result_index + 1] = E[i];
    }

    puts(R);

    return 0;
}