def get_length(fn):
    with open(fn, 'r') as inp:
        for line in inp:
            if line.strip().startswith('HOMOGENEIZED CROSS SECTIONS FOR CELL'):
                line = next(inp)
                line = next(inp)
                line = next(inp)
                sigtr = float(line.split()[9])
                siga = float(line.split()[6])
    length = 1.0/(3.0*sigtr*siga)
    return length