How to change textfield from another textfield

Hello, i have 2 textfields, textFieldOne and textFieldTwo.
How to change textFieldTwo, every time when i enter number in textFieldOne, without button to change.
Suppose, we enter "1" in textFieldOne and textFieldTwo displays it like "1" * 10 = 10(for example) i.e. and if we enter "12", textFieldTwo displays "120"
I hope the logic is clear, thanks in advance for the answer