Demonstrate that if a call at a lower strike of K1 is less expensive than a call at a higher strike of K2, a call spread where one shorts the higher strike call and goes long the lower strike call will always make money for the seller. (This is referred to as call spread arbitrage.) Derive the equivalent result for puts using put-call parity and discuss the meaning of the arbitrage in terms of the implied cumulative distribution of the stock at the relevant maturity.