sed command to delete line

HI,

Im using the below commmand to delete lines having "(" at the start of a line.

 sed -e  '/^(/d' 

But i need a command to delete lines that has only "(" in it and no other strings.

infile -

asdf
asdf
( asdf
(
asd

outfile

asdf
asdf
( asdf
asd

Thanks

sed '/^($/d' inputfile