sum formula in crystal reports

Question

    Hello everyone !
I have problem in sum formula in crystal reports.I have one field in my report "Completion time" , every task have Completion time.In the end,i want total time,
i use formula like this , Sum({Completion_Time}),but when i drag it,to my report, its not working. Can anyone help about it, how to make sum formula in crystal reports.Pls help, its urgent.