A random sample of size 4 is taken from the distribution with the probability density function
\[
f(x; \theta) =
\begin{cases}
\frac{2(\theta - x)}{\theta^2}, & 0<x<\theta, \\
0, & \text{elsewhere}.
\end{cases}
\]
If the observed sample values are 6, 5, 3, 6, then the method of moments estimate (in integer) of the parameter \( \theta \), based on these observations, is ________