阅读理解，判断正误。
Bill and Ben are shopping.
Bill: There is a store. Let's go in.
Ben: Let's go.
Assistant: Can I help you?
Bill: Yes, do you have any shoes?
Assistant: Yes, we do. What size do you want?
Bill: Size 21, please.
Assistant: Here you are.
Bill: Oh, this shoe is just right for my left foot. But this one is too big for my right foot.
Assistant: You can try this pair on. They are size 20.
Bill: Oh, this time my right foot is all right. But this shoe is too small for my left foot.
Assistant: What size do you want?
Bill: I don't know. Can you give me an idea?
