Wi-Fi network key must be 5 or 13 characters!

Hello,

I'm using an Acer Aspire 4720 laptop. I went to the public library for wireless internet access but when I keyed in the password which the librarian gave me which is "public" to connect to the internet, it appeared an error message saying that the network security key is not valid and the password must be either 5 or 13 characters.

Is there a way to change the settings so that I can enter "public" which contains 6 characters?? cos I don't think it's possible to ask the entire public library to change the password just to accommodate me..