always get absolute value of unknown input data (sometimes positive/sometimes negative)

Question

I need to find an absolute value for a variable, but it is homomorphic encrypted. So, sometimes my x might be x=2, or other times x=-4. I want my function to always return the positive value even if the input is positive or negative, without knowing how it actually is. I can't do comparison. All i can do is add/sub/mult with my variable x, and it's negation. Can you think at a mathematic way to build a function that always returns me the positive value?? Thank you!