Given is the function $f(x)=\lfloor 2^{30.403243784-x^2}\rfloor \times {10}^{-9}$ ($\lfloor \rfloor$ is the floor-function),
the sequence $u_n$ is defined by $u_0=-1$ and $u_{n+1}=f(u_n)$.

Find $u_n+u_{n+1}$ for $n={10}^{12}$.
Give your answer with $9$ digits after the decimal point.