Getting Errors I Don't Understand Within Create Function

Here is the function I'm trying to write. The purpose is to replace MS Access Val() function. I'm not finished with the logic, I'm just trying to get this much to work now. Here is the function:create function DBO.NumValue
-- This function will get the numbers from the front of a field
-- and return the value of those numbers in a numeric data type
(@mNumInput as charvar(100))
RETURNSnumeric
AS
BEGIN
declare @x as tinyint
declare @x1 as tinyint
SET @x = 1
WHILE IsNumeric(SubString(@mNumInput, @x, 1))
BEGIN
SET @x1 = @x
SET @x = @x + 1
CONTINUE
END
If @x1 > 0
BEGIN
RETURN CAST(LEFT(@mNumInput, @x1), Numeric
END
END
Here are the two error messages I'm getting from this function.Server: Msg 156, Level 15, State 1, Procedure NumValue, Line 12
Incorrect syntax near the keyword 'BEGIN'.
Server: Msg 156, Level 15, State 1, Procedure NumValue, Line 20
Incorrect syntax near the keyword 'END'. I have no idea what these two error messages mean.
TIA,