A photon of energy $E$ collides with a particle of rest mass $m$, which is at rest. The final state consists of a photon and a particle of rest mass $M, M>m$. Show that the minimum value of $E$ for which it is possible for this reaction to take place is

$$E_{\min }=\frac{M^{2}-m^{2}}{2 m} c^{2}$$