Highlight Column C and Edit | Insert to create a new column C. Enter
this formula in the new C3:

=A3 & B3

and copy down to C11. Amend your formula in what is now D18 (used to be
C18) to:

=VLOOKUP(A18&B18,$C$3:$D$11,2,FALSE)

and copy this down as necessary. You can hide the new column C if you
want your sheet to look like it did before.

Hope this helps.

Pete