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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_uptime(self, app_stats):
 """ Return Icinga's uptime """

        if "program_start" not in app_stats.keys():
            return 0

        if not app_stats["program_start"].isdigit():
            return 0

        uptime = int(time.time()) - int(app_stats["program_start"])
        if uptime < 0:
            return 0

        return uptime