10 good friends live far away from each other and have no telephone, so they can only communicate with each other by writing letters. Each of these 10 people knows one piece of good news (these 10 people know different good news), in order to let these 10 people know all the good news, how many letters should they let the postman deliver at least?
10个好朋友彼此住的很远，又没有电话，只能靠写信互通消息．这10个人每人知道一件好消息(这10个人各自知道的好消息不同)，为了让这10个人都知道所有好消息，他们至少让邮递员送几封信？
<gadget id="calculator">9 + 9</gadget>
<output>18</output>

<result>18</result>