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Rule $\qquad$$\newline$\begin{tabular}{|c|c|}$\newline$\hline Input & Output \\$\newline$\hline $1$ & $7$ \\$\newline$\hline $2$ & \\$\newline$\hline $3$ & \\$\newline$\hline $4$ & $13$ \\$\newline$\hline$\newline$\end{tabular}