Sequences

An arithmetic sequence is one in which there is some first number, and then a series of numbers which are all a fixed number different.

For example 3, 5, 7, 9 is an arithmetic sequence that has a first number 3. Then each term after that in the sequence is formed by adding 2 to the previous term. (The terms are different by 2). The 3 is also called the first term (term 1) and 9 is the 4th term.

Given a starting number, a difference and a value, your program is to work out if the number could be part of the sequence. If so, output which term that number would be, and if not, output a letter X.

Input will consist of a number of lines, where each line has 3 numbers separated by spaces.

The first number is an integer that is the first term in the sequence. The second is the difference - this will be a non-zero integer. The third is the value that you will need to test to determine whether it can be part of the sequence or not.

Input is terminated by a zero value for each of the 3 numbers.

Output will consist of one line for each input line. It will consist of either a number indicating which term it is, or X if the number isn’t part of the sequence

• -6,000,000 ≤ a starting number, a difference, a value ≤ 6,000,000

11 is the 5th term

The sequence is –1, -4, -7, -10

(-1+ -3 = -4, -4 + –3 = -7, -7+ -3 = -10)

-8 isn’t in the sequence.