Reset variable in each grouping

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CrystalReports

my greeting for all,
i have crystal report with grouping, i use variable to make some
calculation into report , i want to reset this variable at the end of each group

ex: variable name var1 in formula name balance i use

numbervar var1=0;
 var1:=var1+(such column variable) 
and the report aspect as this :
(group)name1
  ----------
        child1        var1(10)
        child2        var1(20)
---------------------------------------------------------------
i need to reset var1 by 0 here to start again 
(group)name2
------------
   child1         var1(10)
   child2         var1(20)
.....

i hope that is clear
thank all