keypad lcd and arithmetic help

i have a pic with an lcd and keypad. i need to perform a simple calculation based on 4 different variables which are numbers entered on the keypad and displayed on the lcd. i need to be able to input numbers of different values sometimes 100s and 10s and thousands. how can i get the pic to understand the overall value of the number entered? how does 1 2 3 become 123 in the pics mind? if 123 is displayed on the lcd can i use the lcdin command to send it directly to the pic and have it used in my calculation? is an array the correct thing to do? any help or examples is greatly appreciated.

jordan