Extraction I: Secret Message in the Noise
A spy has hidden a secret message inside a string text of random characters. Your task is to extract the hidden message by pulling out every step-th character.
Input: Two arguments. String (str) and positive integer (int).
Output: String.
Examples:
assert extraction("hxeqlzlzow!", 2) == "hello!"
assert extraction("abcdefr", 2) == "acer"
assert extraction("dbcoefc", 3) == "doc"
Preconditions:
- step > 0