A, B, C, and D compare their ages. If A is 3 years older than C, B is 3 years older than D, A is 2 years older than D, and B is 4 years older than C, then how old is A younger than B?
甲、乙、丙、丁四人比年龄．如甲比丙大3岁，乙比丁大3岁，甲比丁大2岁，乙比丙大4岁，那么甲比乙小多少岁．
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