Summation of the values

Hi All,

Would like to have a sum of the below three nodes at the bottom of the node with name Total under the Classification Object

Total.

Classification              Amount

Service (HeirNode)            100

Breakfix(Heir)               -50

Medium (Heir)                -50

Total                             0

The Total should be sum of three nodes ..How can i display the Total as Node  under the  Classification Object  with summation = 0 in the report.?

Thanks