Atomic Structure

In the two elements, ${ }_{Z_1} A^{M_1}$ and ${ }_{Z_2} B^{M_2}$, the following relations are true. $M_1 \neq M_2$ and $Z_1 \neq Z_2$ but $M_1-Z_1=M_2-Z_2$. These elements are ( $M$ is atomic weight, $Z$ is atomic number)