Digits Only Please

Ask How many tickets? (with a space after the question mark) and keep asking, printing Digits only. each time, until the stripped answer is made of digits only, checked with .isdigit(). Then convert it with int() and print Booked <n> tickets.. Input: one answer per line; the last line is always digits. Each prompt is part of the output, so the message printed after a bad answer, and the final sentence, follow a prompt on the same line.