Hi I´m new in salesforce and I need to do a rollup summary in a custom object, it should call the value from a formula field of the child object, but when I try to do the sum it does not show the field that I need. Could I create a code or trigger or something to do it manually? Thank you.

Hi I´m new in salesforce and I need to do a rollup summary in a custom object, it should call the value from a formula field of the child object, but when I try to do the sum it does not show the field that I need. Could I create a code or trigger or something to do it manually? Thank you.