<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def surface_area(self):
 """ Calculates the surface area of the slab """

        m = self.lattice.matrix
        return np.linalg.norm(np.cross(m[0], m[1]))