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?