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

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|lambda

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

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