Lookup Next Blank Cell and Return Adjacent Cell

Good morning all,

Apologies if this has been asked before, but I cannot find any reference to what I need on google.

I am trying to create a formula that will lookup the next blank cell in column B and return the corresponding cell from column A. I cannot fathom this one out!

So in the next example, I would like the formula to return the number "4".

A | B
1 | Excel
2 | Word
3 | Office
4 |