def log_evidence_from_file_summary(file_summary, prior_count):
    """Open the file "multinestsummary.txt" and extract the log evidence of the Multinest analysis.

    Early in the analysis this file may not yet have been created, in which case the log evidence estimate is
    unavailable and (would be unreliable anyway). In this case, a large negative value is returned."""
    try:
        with open(file_summary) as summary:
            summary.read(2 + 112 * prior_count)
            return float(summary.read(28))
    except FileNotFoundError:
        return -1e+99
