Cool. Just so you know Type N is a character type also, where as type I is a true numeric type. So just like a character you need to define a field with the size parameter unless it is size 1.
i.e.
data: v_mon(2) type n. " For a 2 character number
However if your intention is just to use it as a number then stick to Integer type.
I'd recommend you check out the SAPHelp on datatypes and familiarise yourself with the different types and how they work. That will help you a lot to avoid stuff like this.
Cheers,
Katan