What is the grammar for language $L = \{ a^nb^m : n!=m-1\}$?

I only know I have to write grammar for both $ n<m-1 $ and $ n>m-1 $, so this is what I wrote:

for n<m-1
S->Abb
A->aAb|landa

for n>m-1
S->aaA
A->aAb|landa

Yet I can not mix them and get a correct grammer for it.