Populate field value based on another field input

Populate field value based on another field input

I have two fields
- a dropdown field "dropdown1"  with two values: "valueX" and "valueY"
- a numeric field "numeric2" which should be populated with value "1" if "valueX" is selected in "dropdown1" or should be populated with value "2" if "valueY" is chosen in "dropdown1" field.
Is there a script which can solve this problem? Where it should be placed on (on user input, on success, etc)
Thank you!