Newbie question - thanks for any help.

I have the following:

31
2
4
4
2
3
61
9
3
2
31
2
3
4

And would like a formula to number the rows until the next 1 occurs. So in the end, I want it to look like:

31
22
43
44
25
36
61
92
33
24
31
22
33
24

It's about 500,000 rows long, so any formula shortcut would be appreciated.

Thanks!