Problem:

Columns A & B contain numbers and matching letters.
However, rather than being adjacent, each letter in column B is shifted down one row with respect to its matching number in column A.
We want to retrieve the matching letter for each number.

Solution:

Use the INDEX and MATCH functions as shown in the following formula:
=INDEX($B$2:$B$7,MATCH(C2,$A$2:$A$7,0)+1)