How to do arithmetic operations in Virtual datapoints

Below link contains the expressions which can be used in defining the Virtual Datapoints
https://www.logicmonitor.com/support/datasources/creating-managing-datasources/datapoint-expressions/

This site has the format how to use the arithmetic operators also. I found the expression for the addition is "x+y".

I assumed 'x' is the 1st datapoint and 'y' is the 2nd datapoint. So used my datapoints in the same way. But I have not got any results for the expression, only the error message saying "Invalid virtual datapoint expression".

Could anyone please help me on this?