There is a formula =h1-f1 but what about the text or give priority to f1 than h1?

Question

Google Sheets

There are 3 columns 1 is {mm} 2 =h1-f1 3 {mm} The total for 2 columns is {mm}
It happens that 3 columns have the word {3 The column updates itself} I need the formula to give priority to words and not give an error that words cannot be used.

Answer 1

Might fit
=IFERROR(C2-A2;E2)