Suppose that $c_1, c_2$, and $c_3$ are the prices of European call options with strike prices $K_1$, $K_2$, and $K_3$, respectively, where $K_3>K_2>K_1$ and $K_3-K_2=K_2-K_1$. All options have the same maturity. Show that
$$
c_2 \leqslant 0.5\left(c_1+c_3\right)
$$