Investigating the behaviour of a recursively defined sequence (noch nicht übersetzt)

Given is the function f(x) = ⌊2^{30.403243784-x2}⌋ × 10^-9 ( ⌊ ⌋ is the floor-function),
the sequence u_n is defined by u₀ = -1 and u_n+1 = f(u_n).

Find u_n + u_n+1 for n = 10¹².
Give your answer with 9 digits after the decimal point.