How would one include the following conditions in a folmula

IF INPUT worksheet Column A= A1 and INPUT worksheet Column B =B1 then C1=1
IF INPUT worksheet Column A= A2 and INPUT worksheet Column B =B1 then C1=2

IF INPUT worksheet Column A= A1 and INPUT worksheet Column B =B2 then C1=4
IF INPUT worksheet Column A= A1 and INPUT worksheet Column B =B3 then C1=5

etc...