It was an oral English class and in front of us was standing our teacher Lila Betray, who was a university student from the US. Somehow we came to the point whether any of us could sing an English pop song in front of the class. You know, maybe three or more out of ten Chinese students are shy in class. As a result, there was silence in class for quite a while, which made everybody feel rather embarrassed (尴尬的), including the teacher.

    I was a shy boy. However, just at that moment, a great idea struck me suddenly. “Seize the enjoyment! Seize the chance. Yes, there couldn't be a better chance.” I said to myself. “Why not seize it? Why should I have always to be the guy regretting missing the opportunity just because of shyness? Yes, maybe I should have a try!”

    So, I raised my hand. And I remember that it took me quite a while to think what the song's name was. Though I sang no more than five sentences because of forgetting the lyrics (歌词), their clapping hands told me that I did the job well. Later, I got a gift and I was really happy.

    I think that is one of the most unforgettable moments in my life. Through this experience, I've also got to know that we should believe that we can do the job well, no matter how difficult it seems for us. Just have a little more courage.

设f（x）= $\{\begin{array}{c}\sqrt{1-x^2},x\in -1,1)\\ x^2-1,x\in 1,2\end{array}$ ，则 $\int {}_{-1}^2$ f（x）dx的值为（   ）