How to link two parameters in Catia?

Hello,
Suppose i make a pipe with two parameters length and dia such that the length is twice as dia
Now how do i link both the parameters such that whenever i make change in one the other also change
I did (length=2*dia)
But that means length depends on dia
If i change dia length changes
But i want vice versa
By this method i cannot enter the value of length i can just control dia
So what if i want to enter either of the value the other gets updated