Determine the lateral surface area of the right cut square pyramid from the base of \textbf{a}, \textbf{b} and height \textbf{h}. \InputFile In a single line through the gap by reason of the pyramid \textbf{a}, \textbf{b} and height \textbf{h}. All input data are integers, not exceeding \textbf{100}. \OutputFile The search area, the result output to two decimal places.