Pandas dataFrame using groupby but repeating non sequential values

Question

I m new to pandas, and need the following : my DataFrame looks like

'a'    'b'
S1      4
S1      7
S2      1
S2      3
S2      9
S1      4

I d like to group it by column a, but grouping only subsequent lines, so the result of calling count on the groupby would get me something like

S1   2
S2   3
S1   1

Is there an option or another function to achieve this ? Thanks