Swift 3 : Decimal to Int

This is my updated answer (thanks to Martin R and the OP for the remarks). The OP's problem was just casting the pow(x: Decimal,y: Int) -> Decimal function to an Int after subtracting 1 from the result. I have answered the question with the help of this SO post for NSDecimal and Apple's documentation on Decimal. You have to convert your result to an NSDecimalNumber, which can in turn be casted into an Int:

let size = Decimal(2)
let test = pow(size, 2) - 1
let result = NSDecimalNumber(decimal: test)
print(Int(result)) // testing the cast to Int

let decimalToInt = (yourDecimal as NSDecimalNumber).intValue

or as @MartinR suggested:

let decimalToInt = NSDecimalNumber(decimal: yourDecimal).intValue