How to get sum by groups in Crystal Report

I want to get the sum by the groups in Crystal Reports.

Group OVERALL
-----------------
      group 1
     --------------
          5.00
          5.00
         10.00
     --------------
     total group 1      20.00
     --------------
      group 2
      --------------
         10.00
         25.00
         15.00
      --------------
     total group 2      50.00
-------------------
Total Group OVERALL     70.00

Using this formula sum({field.amount}, {field.group}) I only get sum for OVERALL but not for group 1 and 2.