How to perform data manipulation in report?

I need to subtract 2 values from one to get the resultant in my report. How to get this.

Subtract [Sum(X)] and [Sum(Y)] from AB_Total

Kindly sort this out

Solution

Retrieve the data fields into a dataset(dataset1). Put the expression below in to a textbox:

=SUM(Fields!X.value,"dataset1")-SUM(Fields!Y.value,"dataset1")