= Call option on a put option
{title2=$[p(T_1,S_{T_1};K_2,T_2)-K_1]^+$}
This compound <European contingent claim> gives the right to buy a later-expiring <put option> at time $T_1$ for $K_1$. Its payoff is the positive part of the put value minus this exercise cost. Under the <Black-Scholes model> the put value decreases strictly with the stock spot, so a nondegenerate positive exercise cost gives a unique upper spot threshold for exercise. With zero exercise cost, the compound claim has exactly the underlying put's price by the <tower property>.
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