Term index in an arithmetic sequence

An arithmetic sequence is a sequence in which the difference between two neighboring terms is always the same. That fixed difference is called the common difference.

• 1, 3, 5, 7, 9, ...
• 2, 4, 6, 8, 10, ...
• 1, 5, 9, 13, 17, ...
• -1, -3, -5, -7, -9, ...

The sequence with first term $a$ and common difference $d$ runs $a$, $a+d$, $a+2d$, $a+3d$, so its $n$-th term is $a+(n-1)d$.

Write a program that finds which term of that sequence the number $k$ is.

The first line contains three integers $a$, $d$, $k$ separated by spaces.

• The first number is the first term $a$, with $-1000 < a < 1000$.
• The second number is the common difference $d$, with $-1000 < d < 1000$ and $d \neq 0$.
• The third number is the value $k$ you are looking for, with $-1000000 < k < 1000000$.

Print one integer, the position of $k$ in the sequence. The first term counts as position 1.

If $k$ does not occur in the sequence, print X.