How do you create an inverse sequence in R

Question

I Have a sequence below that has a pattern that does not change. I can create a vector to represent the missing variables to this pattern(below). But I can't seem to figure out a way to print this specific sequence below as a vector. How would one create a vector that shows the sequence below but stops at a specific row(the last row in the pattern), instead of 51? Thanks

bad <- seq(1,51,by=3)

2,3,5,6,8,9,11,12,14,15,17,18,20,21,23,24,26,27,29,30,32,33,35,36,38,39,41,42,44,45,47,48,50,51

Answer 1

cumsum(rep(c(2,1), 51/3))

probably inefficient though.