Find number abc

Britney Spears and Christina Aguliera were building a barn. While putting up the inside wall, Britney noticed that Christina was tossing every other nail into the garbage can.

Britney asked Christina, "Why are you throwing those nails away?"

Christina said, "The pointed end is on the wrong end of the nail."

Britney said, "Well, don't throw those away, we can use those on the outside wall!"