I created 3 variables which i used to do calculations
i called one numberOfNights = Name , Value = numberOfNights type = String, the second one perdiem = Name , Value = perdiem type = String and the last one
is a javascript expression i used for some calculations
Now i used some Javascript function like this :
var CalculateAmount = Number($data.perdiem) * Number($data.numberOfNights);
return CalculateAmount
when i created another variable of Type Javascript Expression with name PerdiemCalculateDays , now i save.