duplicate field value and increase it by one

Good day,

I'm true Adobe and Javascript novice and I have been trying to come up with a way to duplicate field value (Year, yyyy) and increase it by one. So if user enters 2013 this field called Year3 should say 2014. I have duplication going using: getField("Year2").value =getField("Year").valueAsString;

but I don't know how to get the field Year3 to go one up. I tried formatting it as string or number and do just + 1 but that makes it 20131.

Thanks

Amko