How to create polygon which shape are area that input by user using google map v3?

I would like to create polygon which shape are area that input by user. For example, user input Chicago. Then map will show polygon like below image :

I cant show image because i dont have enough reputation

Is anybody know how to do it? I don't know to get latitude and longitude of edge the area that nee开发者_JAVA技巧ded to draw polygon?

Thanks