Hi everyone.
Ive this problem in measure theory, but I dont know how to treat it.

Let (X,A, \mu ) be a measure space where \mu(X) = 1 and for all E \in A, \mu(E) = 0 or  \mu(E) = 1. Let f: X \rightarrow \mathbb{R} be a measurable function. Then exists c \in \mathbb{R} such that f(x) = c almost everywhere.
Thanks for your Help