Transact SQL :: Split Half Hour In Number From String

How I can split the half hour in number from this string?
 
20130329070000 (it's varchar)
Easy for the date
cast (left(20130329070000,8)as date)

I can achieve even the Half Hour

select
left(SUBSTRING('20130329070000',9,4),2)+':'+RIGHT(SUBSTRING('20130329070000',9,4),2)

But what I need is the Half Hour in numeric format, so 00:00

should be 1, 00:30 should be 22, 01:00
should be 3, 01.30 should be 4......23:30 should be 24.

I already did but I can't find the piece of code I used.