How do I keep a particualr value fixed when using fill.

e.g. I want to use fill to produce a set of formulas:

=A1+K1, =A2+K1, =A3+K1..........

But using fill I get:

=A1+K1, = A2+K2, =A3+K3...........

So how do I do it so that K1 is fixed?