Investigating the behaviour of a recursively defined sequence

Given is the function $f(x) = \lfloor 230.403243784-x^2\rfloor × 10^{-9}$ ( $\lfloor\ \rfloor$ 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.