Suppose that $c_1$ and $p_1$ are the prices of a European average price call and a European average price put with strike price $K$ and maturity $T, c_2$ and $p_2$ are the prices of a European average strike call and European average strike put with maturity $T$, and $c_3$ and $p_3$ are the prices of a regular European call and a regular European put with strike price $K$ and maturity $T$. Show that $c_1+c_2-c_3=p_1+p_2-p_3$.