Let $f(x)=cx(x-1)$, where $c$ is a positive real number.  We use $f^n(x)$ to denote the polynomial obtained by composing $f$ with itself $n$ times.  For every positive integer $n$, all the roots of $f^n(x)$ are real.  What is the smallest possible value of $c$?

Let's think step by step and output the final answer within \boxed{}.
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