How to store input for an unknown number of iterations?

I am writing a problem for school project on loops.

The program is a do-while loop where the user will feed the program sales figures, then expense figures for three separate districts. Then the user will be prompted to enter Y to enter the figures for another month, or anything else to terminate the loop.

After terminating the loop the program will then take the input figures for however many months the user has decided to evaluate and average them and compare them.

How do I store data for additional months and still keep the data for the first month?