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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_run_number():
 """ Get a run number for this execution of the model system, for identifying the output hdf5 files). Returns ------- The integer number for this run of the model system. """

    try:
        f = open(os.path.join(os.getenv('DATA_HOME', "."), 'RUNNUM'), 'r')
        num = int(f.read())
        f.close()
    except Exception:
        num = 1
    f = open(os.path.join(os.getenv('DATA_HOME', "."), 'RUNNUM'), 'w')
    f.write(str(num + 1))
    f.close()
    return num