[fpc-pascal] Convert to int64 note

[james at productionautomation.net]

I've been trying to get my programs to compile without any warnings or notes, but I keep getting this one:

promill.pas(2137,42) Hint: Converting the operands to "Int64" before doing the subtract could prevent overflow errors.

I get it a lot, but this is the line the above example is from:

Last_GS:=G_End-(Max_Program_To_Display-1);

Can someone please explain that this hint means and how I would convert the operands to "Int64" as recommended?

James