Backup of a cracked screen

I cracked my iPhone 5 screen and the right side of the screen is black and un-touchable but the left is fine. I want to back my phone up to my computer and when i try iTunes says "iTunes could not back up the iPhone because it is locked with a passcode. You must enter your passcode on the iPhone before it can be backed up." but i can't put my password because i can't touch all the buttons to put it in. HELP!!!