To Create a Valid Expression to Define a Variable

Open the Expression Builder dialog from the Variables dialog. If the parameter already had an expression defined, it would appear in the Expression Field.
Enter the constituent elements for your required expressions (eg. other existing variables, functions, operators, etc.)
Note:
1. Double-click a variable, function, or constant from the Symbol Type Selector to insert it at the cursor position in the Expression Field.
2. Click an operator symbol in the Toolbar above to insert it at the cursor position.
3. Tooltips of functions and constants tell you the arguments and return value.
(Optional) Select a part of the expression and click () in the toolbar, to group or ungroup it.
Click Test or press enter to parse the built expression. If an error is found, it will be reported and the cursor will move to the position at which the error occurred:

Note: In case of an error, OK will be disabled.

Fix the error, and then click Test again, and the OK button becomes enabled:
Click OK. The selected variable (in the Variables dialog) is now defined by this built expression.