Unbiased estimator through the maximum

Let $X_1, X_2$ be a random sample from a distribution having the population density function $f(x) = \begin{cases} 1/\theta, & 0 < x < \theta \ 0, & \text{otherwise} \end{cases}$, where $\theta \in (0, \infty)$. Let $X_{(2)} = \max\{X_1, X_2\}$ and $\psi(\theta) = P_\theta(X_1 + X_2 < 1)$, $\theta > 0$. Let $\delta(X_{(2)})$ be an unbiased estimator of $\psi(\theta)$ that depends on observations $X_1$ and $X_2$ only through $X_{(2)}$. If $\delta(t)$ is a continuous function on $(0, \infty)$, then the value of $18 \delta(3/4)$ equals ________ (answer in integer).