I Want a value of a variabel to not exceed an other value of an other variable

I Want a value of a variabel to not exceed an other value of an other variable

How to make the value of the variable in one incremental step dont exceed the value of an other variabel in other incremental step.

Exemple:
My user willl select ther max value and the minimum value, I dont want the system to let him put the minimum bigger ou equal the maximum.

There is any way to do this ?