create function dbo.valid_tz (@tz_str varchar(20))
returns int
as begin
	
	declare @pos int = 1
	declare @tested_char char
	declare @char_score int
	declare @total_score int = 0
	declare @result int

	while @pos <= len(@tz_str)
		begin
			set @tested_char = SUBSTRING(@tz_str, len(@tz_str)-@pos+1, 1)
			if @pos%2 = 0
				begin
					set @char_score = cast(@tested_char as int)*2
					if @char_score>9 
						set @char_score = 1+@char_score%10
				end
			else
				set @char_score = cast(@tested_char as int)
			set @total_score = @total_score + @char_score
			set @pos = @pos + 1
		end

	if @total_score%10 = 0 
		set @result = 1
	else
		set @result = 0

	return @result

end