On one of the secret plants is processed radioactive materials, which resulted in radioactive waste in two types: type \textbf{A} (very dangerous) and type \textbf{B} (non-hazardous). All waste is packed in special rectangular containers of equal size, after which the containers are stacked in a pile on top of each other to save. A stack is explosive if it side by side two boxes of waste by type \textbf{A} is required to write a program that counts the number of possible formation of non-explosive stack of a given total number of containers, \textbf{N}. \InputFile The input file contains a single integer \textbf{N} (\textbf{1} ≤ \textbf{N} ≤ \textbf{100}). \OutputFile The output file must take the required number of options.