Programming a calulator

How can I hack this cheap solar-powered calculator that the bank gave me for Christmas? It has, like, four bytes of memory and can display seven numbers on the screen. It has no interface at all, so I'll probably have to dust off the old soldering iron and put a serial bus in. Ideas on where to begin?

>>2
You are locked in a large room with a large quantity of food and water. You have a large number of 74xx ICs (as well as their datasheets) and a few calculators like the one described in >>1. The exit door is locked with a password. The password is the sum of the first 1000 prime number.