def get_atom_num(ndx_file: str) -> int:
    """Computes number of atoms in the particular index file.

    Args:
        :param str ndx_file: .ndx - index of the protein atoms of the current conformation.

    Returns:
        :return: number of atoms in the .ndx file.
        :rtype: int
    """
    with open(ndx_file, 'r') as index_file:
        index_file.readline()  # first line is the comment - skip it
        indices = index_file.read().strip()
    elems = indices.split()
    atom_num = len(elems)
    return atom_num