Efficient Inverse of a updated lower triangular matrix

Dear All,

I am looking for the inverse of a updated lower triangular matrix. The original matrix is A which is a lower triangular matrix. Now I am looing for the inverse of A+B, where B is a zeros matrix except that two rows of B have 2 non-zero elements in each row. A+B is also a lower triangular matrix.

Thanks a lot in advance.

Benson