Excel cell reference

Q-Actually I have a list of numbers from A1 to A100. Out of this list I want to choose the numbers say from A20 onwards.
But this need to be done through formula. Like, I have 'A' written in Say C1 and 20 in D1. Now I want to combine A and 20 to get
the value of A20 cell. How to do it?

Thanks in advance-Naveen