Grades four, five and six in a certain school organized a theatrical performance with 18 performances in total. If each grade performs at least four programs, how many different situations of the number of performances of these three grades are there?
某学校四、五、六三个年级组织了一场文艺演出，共演出18个节目．如果每个年级至少演出四个节目，那么，这三个年级演出节目数的所有不同情况共有多少种．
<gadget id="calculator">3 * 4</gadget>
<output>12</output>

<gadget id="calculator">18 - 12</gadget>
<output>6</output>

<gadget id="calculator">1 + 2 + 3 + 4 + 5 + 6 + 7</gadget>
<output>28</output>

<result>28</result>