Solutions for ODE with periodic B.C.

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I have the following ODE $$-u(x)''=f(x) \qquad u:[0,1)$$ It is smooth and 1-periodic. Assume that I have a solution u(x). How do I prove that: $$u(x)= u(x)+c \quad c \in \mathbb{R} $$ is also a solution. It is obvious by just looking at it. But how do I approach a proof?