formula

I have a problem witha spreadsheet in excel 2010 I have 3 columnsin sheet 1 and 1 column in sheet 2 the columnsare named in sheet 1 stock level , addition , total stock in sheet 2 the column is named picked, I want the figure in total stock columnto be the result of a formula eg = stock level + addition - picked and leaving the calculation in total stock column and this figure will only change when I add stock it always depletes from the picking figure can someone help
Thank you

your question is not very

your question is not very clear. Try to be more specific. Do you want a formula that only calculates under certain conditions?