def _longest(ts):
    if ts == []: return 0, 0

    ldepth, lpath = _longest(ts[0])
    rdepth, rpath = _longest(ts[2])

    depth = max(ldepth, rdepth) + 1
    cpath = max(lpath, rpath)

    return depth, max(ldepth+rdepth, cpath)

def longest(ts):
    return _longest(ts)[1]