Calculating values based on other inputs.

So, I know that this is about as basic as it gets but I could use some help. I'm creating a worksheet which allows the user to input a number of different values. Within this larger group of inputs, there exists 4 inputs which only require 2 to be known to know the remaining 2. These 4 inputs are R, CP, CV and Gamma. Given by:
R = CP - CV (1)
Gamma = CP/CV(2)
CV = R/Gamma - 1 (3)
CP = R + CV (4)

As you can see, if any two of the above are know, the other 2 can be calculated. I've tried the following
Column A _______Column B
R ____________= CP - CV
Gamma________= CP/CV
CV___________ = R/Gamma - 1
CP____________ = R + CV

This throws up a problem though. It says I've created a 'circular' reference and then when I input values for, let's sat, Gamma and CP, the remaining 2 aren't calculated. How do I fix this so that when I input any 2, the remaining 2 are calculated? Thanks.(Note, I've named the cells corresponding to CP, CV, R and Gamma)