At the factory each of the \textbf{N} items can be processed in one of two machines: \textbf{A} or \textbf{B}. Each piece has a serial number from \textbf{1} to \textbf{N}. The processing of parts after the other, in accordance with their numbers. Amount of detail is always even. There are rules under which determines whether you can handle detail on a particular machine. \begin{enumerate} \item If currently on the machine \textbf{B} was processed the same number of parts, as well as on the machine \textbf{A}, then the next part must be machined \textbf{A}. \item In sum, in each of the machines should be treated the same amount of detail. \end{enumerate} How many people, so many opinions. Each of the employees of this plant has offered a sequence of machining, all proposals have been different, but those that satisfy rules \textbf{1} and \textbf{2}. Write a program that according to information on the number of parts N determines the maximum number of factory workers. \InputFile One even integer \textbf{N} (\textbf{2} ≤ \textbf{N} ≤ \textbf{28}) - the amount of details to be processed. \OutputFile Print one integer - the maximum number of factory workers.