Nick was born with no arms or legs. There was only a small foot on his hip(臀部). He can't do the easiest things like washing his face or brushing teeth, but he can type, pick things up between his toes (脚趾)and even kick a ball.

    Nick grew up in Melbourne, Australia. He wasn't happy at school because his classmates often laughed at him. At the age of 10, he even wanted to kill himself. But later, he changed. He tried to look at good things around him. When he was 17, a friend encouraged(鼓励) him to give speeches. From then on, he began to travel around the world and give talk about his life. He thinks it can help others. Up to now, he was been invited to more than 50 countries and given thousands of talks.

    Now Nick is 32 years old. He lives in California with his wife, Kanae, and their 2-year-old son.

    When you are sad, think of him. At least you have arms and legs.

观察图中的图形，并阅读图形下面的相关文字：

三角形的对角线有0条，四边形的对角线有2条，五边形的对角线有5条，六边形的对角线有9条．

通过分析上面的材料，请你说说十边形的对角线有多少条？你能总结出n边形的对角线有多少条吗？