From May 1st, (on May 1st there are both football and billiards programs) the TV station has a football program every three days, and a billiards program every four days. How many days of the month can there be both football and billiards?
从5月1日起，(五月一日这天既有足球又有台球节目)电视台每三天有一期足球节目，每四天有一期台球节目．这个月有几天可以既有足球又有台球节目？
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