Sequences

An arithmetic sequence is a sequence of numbers in which every term after the first is obtained by adding a fixed number, called the common difference, to the previous term.

For example, $3, 5, 7, 9$ is an arithmetic sequence whose first term is $3$ and whose common difference is $2$: each term after the first is formed by adding $2$ to the previous one. Here $3$ is term $1$ and $9$ is term $4$.

Given a first term, a common difference, and a value, determine whether the value can appear in the sequence. If it can, output which term it is; otherwise output the letter X.

The input consists of several lines. Each line contains three integers separated by spaces:

• the first term of the sequence,
• the common difference (a non-zero integer),
• the value to test.

The input is terminated by a line containing 0 0 0, which must not be processed.

For each input line except the terminating line, print a single line containing the term number of the value, or X if the value is not part of the sequence.

• The first term, the common difference, and the value are each between $-6000000$ and $6000000$.

For the first query 3 2 11, the sequence is $3, 5, 7, 9, 11, \dots$, so $11$ is term $5$.

For the second query -1 -3 -8, the sequence is $-1, -4, -7, -10, \dots$, so $-8$ does not appear and the answer is X.