There are 89 people in classes A and B, 81 people in classes B and C, and 83 people in classes C and D. How many people are in classes A and D?
甲、乙两班共89人，乙、丙两班共81人，丙、丁两班共83人，甲、丁两班共有多少人？
<gadget id="calculator">83 - 81</gadget>
<output>2</output>

<gadget id="calculator">89 + 2</gadget>
<output>91</output>

<result>91</result>