Expected data type BIGINT found dataType STRING

Expected data type BIGINT found dataType STRING

I have an application where i have a drop down field where the options are numbers 1 to 5, a current date field and a return date field, what i want to do is add the number of days selected plus the current date field and set them as the return date date field.

I had success using the addDay function and a static number 
This works:
  1. ReturnDate = input.Delivery_Date.addDay(3);
    input.Return_Date = ReturnDate.toDate();

However because i need to use a dynamic number based off a field what i actually need is this
This doesn't Work:

  1. ReturnDate = input.Delivery_Date.addDay(input.Number_Days);
    input.Return_Date = ReturnDate.toDate();

However when i do this the input from the Number_Days field is interpreted as a string and not an integer so i end up with the error below
  1. Error at line number : 1 
    Mismatch data type for function addDay at argument index 1 expected data type BIGINT found dataType STRING
I looked to see if there was a function to convert string to int but i was not able to find one, does anyone know if this is possible?

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