Is there an easy way to get totals by a group number in Excel 2003?

I've a table in Excel like this:

s1010 105
s1010 20
s1013 40
s1014 80
s1014 88
s1014 82

I'd like to add another total column for each number:


s1010 105 125
s1010 20 125
s1013 40 40
s1014 80 250
s1014 88 250
s1014 82 250

I don't know how many lines each number will have or can have - in
other words, S1013 has only one line but next run, it may have 10
lines. Does this make sense? Any easy way to go from first to the
second.