Which number is missing in the white square?

A brunette goes into a doctor's office and says that her body hurts wherever she touches it. "Impossible," says the doctor. "Show me." She takes her finger, presses on her elbow, and screams in agony. She pushes her knee and screams, pushes her ankle and screams, and so it goes on; everywhere she touches makes her scream with pain. The doctor says, "You're not really a brunette are you?" She says, "No, I dyed my hair. I'm naturally blonde." "I thought so," he says. "Your finger is broken."