swift - Hexadecimal calculator -

i´m using swift make base converter, works when base of number 10 or less, when start input letters doesn't work. want input letters , numbers in same variable, take value integer , change base of it. ideas?

@ibaction func input9(_ sender: uibutton) {     let inputnumber = 9      currentnumber = currentnumber * 10 + inputnumber     labeltext.text = "\(currentnumber)" } 

this how input numbers, have no idea how letters