am i the only one who didn't know this?

we all know 18+ is also know as r-rated. so r means restricted and it will be like restricted-rated like there is no age so today i was wondering why the letter 'r' and i started numbering the alphabets.
1. a
2. b
3. c
4. d
5. e
6. f
7. g
8. h
9. i
10. j
11. k
12. l
13. m
14. n
15. o
16. p
17. q
18. r


like r is the 18th letter. am i the only one who didn't know this?