Stable Mex

Chef gives you a sequence $A$ of length $N$. Let $X$ denote the $MEX$ of the sequence $A$. Chef is interested in the count of **positive** values $k$, such that, if every element $A_i$ of $A$ is replaced by $\texttt{max}(A_i-k,0)$, the $MEX$ of the sequence still remains $X$. Find the count of such values. If there are **infinite** such values, print $-1$ instead. As a friendly reminder, the $