I want to reference a cell no matter what happens to it.

Simple example:

From A1 down:
A1=B1
A2=B2
A3=B3

From B1 down:
5
10
15

If I cut and insert B3(15) into B2, the A column would then read as:

A1=B1
A2=B3
A3=B2

How do I keep the A column referencing the original cell?