For a non-dividend paying share of a company whose price at time t is denoted by St, the current price of the share is ${S0 = £100}$. In any year the volatility is $20\%$ (price of the share can either increase by $20\%$ or decrease by $20\%$). The continuously compounded constant annual risk-free interest rate is $r$, such that $e^r = 1.1$
The maturity payoff for a $2$ year derivative contract is
$${S2 \times I(S2 > 90)};$$
(the option striking price is ${£}90$ and ${I(S2 > 90)}$ is the indicator function, i.e. $ I(S2 > 90) = 1\; \text{if} \;S2 > K, 0 \;\text{if} \;S2 ≤ 90)$
How do I determine the current price of the derivative?