Creating a variable in excel

Hello!
Quite not sure how to formulate this but my problem is as following:

Imagine I have the following in an excell sheet

A B
12 A2
12 A6
1
1
2 Sum(value(B1):value(B2))
2
1
1
1
43


I want to be able to in B5 sum any what is written in B1 and B2, so If I change B2 to "A10" it should sum from A2 to A10. So basically I need the values in B1 and B2 to be variables. Hope you understad my question, thankfull for any help.

Regards Alexander