Formula to work out a sum depending on number of weeks

Hi,

I have a spread sheet I have set up to calculate sales results, I have a column for each week in the quarter with the date at the top and a different page for each individual. I want to be able to enter a number to show how many weeks we have had and the spreadsheet to give me a cumulitive amount for the cells up to that date.

For example if I was to enter 1 in a cell A1 I would want the sum t work out just C3 for example.

If I enter 3 in A1 I would want the sum to be C3+C4+C5 wich is the three cells.

If I enter 6 in A1 it would be C3+C4+C5+C6+C7+C8 etc.

Hope this makes sense and someone can help me out,

Cheers,

Malcolm.