Which number replaces the question mark?

A blond and her blond boyfriend went for a walk along the river. The blond walked across alone on a wooden bridge. After crossing the river, the bridge fell down. She called across to her blond boyfriend telling him that she couldn't get back.
He yelled in response, "Wait until dark, and I will shine my flash light across the river. Get on the light beam and walk back."
She replied, "No, I'll get half way across the river, and you will turn the light off on me!"