def rangize_supplement(spans, lngth):
    """
        spans = [(0, 3), (4, 7), (8, 10), (11, 12), (13, 16), (17, 20)]
        rangize_supplement(spans,24)
        
    """
    rslt = []
    si = 0
    ei = spans[0][0]
    if si == ei:
        pass
    else:
        rslt.append((si, ei))
    prev_ei = spans[0][1]
    for i in range(1, spans.__len__()):
        si = prev_ei
        ei = spans[i][0]
        rslt.append((si, ei))
        prev_ei = spans[i][1]
    if prev_ei < lngth:
        rslt.append((prev_ei, lngth))
    else:
        rslt.append((prev_ei, lngth + 1))
    return rslt
