Calculating dnd proficiency bonus using excel?

I'm trying to get excel to automate calculating character sheet numbers for me, what I need is the following:

when I enter 1, 2, 3 or 4 in cell C2, I want the next cell to say 2.

When C2 says 5 to 8 it should equal 3

9 to 12 should equal 4

13 to 16 should equal 5

and finally 17 to 20 should equal 6

Thanks in advance!