Using a helper column (C), put this formula in C2:

=IF(A2<=0,"",COUNTIF(A:A,">0")-COUNT(C$1:C1))

and copy this down to the bottom of your data. Then in B2 you can use this formula:

=IF(OR(C2=MAX(C:C),C2=""),"",A2&"-"&INDEX(A:A,MATCH(C2+1,C:C,0)))

and copy this down.

Hope this helps.

Pete