Let the function $$f:(0, \pi) \rightarrow R$$ be defined by
$$f(\theta) = (\sin \theta + \cos \theta)^2 + (\sin \theta - \cos \theta)^4$$.
Suppose the function 𝑓 has a local minimum at $$\theta$$ precisely when $$\theta \in \left\{\lambda_1 \pi, ...., \lambda_r \pi\right\}$$ where $$0 < \lambda_1 < ... < \lambda_r < 1$$. Then the value of $$\lambda_1 + ... + \lambda_r$$ is _________