Can anyone help with this query?

e.g. I want to work out how many people are working at any time in a
24 hour period. The number of people rostered on differs each day of
the week.

Monday             Time    No. rostered on
        6.00        2
        7.00        2
        8.00        2
        9.00        2
               10.00
        (and so on)

Can I add additional staff number in my 'no. rostered on column' by
just entering the number into the cell.

Say, if there were an extra 2 staff starting their 8 hour shift at 9.
am. is there a way of just typing in '2' into that cell and getting
'4'?

What I do at the moment is enter =2+2 in that cell and the 7 cells
below.  This is awfully tedious when calculating staff numbers for
every hour of a whole week.

Any help on this one would be appreciated.

Thanks very much

Janev