IF calculation

I need a formula that will only give a result if there is data entered in all
cells used in that  formula,eg if a=b-c,& b=20,c=10,then a=10,but if b=20 and
c=blank,then I want a=0.I only want a result in "a" if both "b" and "c" have
data entered in them."c" will mostly not have any data entered for some time
after "b" does,and I want no amount to appear in "a" until "c" has an amount
entered into it.
eg if b=0(or blank),c=10,then a=0(not -10),if b=20,c=10,a=10
I am using excel 2003
thanks for any help I can get

Try this:=if(count(b1:c1)<2,0,b1-c1)Regards,Fred.

Try this:
=if(count(b1:c1)<2,0,b1-c1)

Regards,
Fred.