Swift return values of different types in function based on conditions

I am new in Swift and have a small question.

I want to build a function which can return values of different types based on the conditions. What return type should I use?? Below is the function. I use "??" as the placeholder. Can anybody help to change the "??" with correct return type?? Thanks!!

fun Req_amt() -> ?? {
    if let input_amt = readLine() {
        if let amount = Float(input_amt) {
            return amount
        } else {
            return "Bad number."
        }
    } else {
        return "Bad input."
    }
}