I'm trying to convert text to numbers and failing, can you help?

I've tried this:

=LOOKUP(N2,{"N","3c","3b","3a","4c","4b","4a","5c" ,"5b","5a","6c","6b","6a","7c","7b","7a"},{1,2,3,4 ,5,6,7,8,9,10,11,12,13,14,15,16})

I want the text N in a cell to return the number 1, 3c to return 2, 5c to
return 8, 7a to return 16 etc. I've stared at it for ages and now I'm stuck,
it's not working. Can anyone help?

Many thanks.