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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def contains_vasp_input(dir_name):
 """ Checks if a directory contains valid VASP input. Args: dir_name: Directory name to check. Returns: True if directory contains all four VASP input files (INCAR, POSCAR, KPOINTS and POTCAR). """

    for f in ["INCAR", "POSCAR", "POTCAR", "KPOINTS"]:
        if not os.path.exists(os.path.join(dir_name, f)) and \
                not os.path.exists(os.path.join(dir_name, f + ".orig")):
            return False
    return True