Allow number only, then add static text to final value

Hi,
 
I need some help with custom scripting please. I want to allow the user to enter in a number into a text field. I want the field to automatically add "months" after that number. for example, the user enters the number 60, the final value is "60 months". Can someone out there help me with what I need to enter in the custom calculation script? If it isn't painfully obvious, I'm new to this...