A, B, C, and D participate in a certain computer skills competition. The average score of A and B is a, and the average score of the two of them is 9 points lower than that of C, and 3 points higher than that of D, then the average score of the four of them is a + how many points.
甲、乙、丙、丁四人参加某次电脑技能比赛．甲、乙两人的平均成绩为a分，他们两人的平均成绩比丙的成绩低9分，比丁的成绩高3分，那么他们四人的平均成绩为a+多少分．
<result>1.5</result>