Why enum cannot hold negative decimal in Swift

Question

I'm trying to use enum in my project like this:

enum ShipShift: CGFloat {
    case Forward = 1
    case Stop = 0
    case Back = -1.0
}

but the Xcode throw an error on the third case as

Raw value for enum case must be a literal

then I change my code into:

 enum ShipShift: CGFloat {
    case Forward = 1
    case Stop = 0
    case Back = -1
}

and Xcode is happy.

But I don't know the real reason. Why the enum cannot have a case which raw value is a negative decimal?

What should I do if I want the Back case equal to -0.5?