完形填空
Madison Williams, aged 13, was studying in her bedroom in Dublin, Ohio. Suddenly, her mother, Leigh Williams, rushed in saying1 “A little boy fell into a septic tank（化粪池）, and no one can 2 him.”
They ran to the neighbor's yard, where they found the boy's desperate mother and other people 3a septic tank opening that was slightly wider than a4. The boy, only 2, had slipped in.
Madison quickly 5 the situation. She was the only one who could 6 through the small hole. Without 7she got on her stomach next to the opening, placed her8 out in front of her9 told the adults, “Lower me in.”
Leigh and the others 10her waist and legs. She moved her arms and shoulders from side to side until she got through the 11 Inside, the tank was dark, and the air was sick. She used her hands to 12 the boy in the dirty water, and jammed her left wrist 13a hidden pole, injuring the muscles in her wrist and arm. Ignoring her14 Madison continued. Minutes ticked by before she15grasped his foot.
Then, ten minutes after Madison had16 the tank, she and the boy were17. But the boy wasn't out of 18.He had been kept from oxygen long enough that he wasn't19. An adult gave him several hard hits on the back. It was only when Madison heard him cry that she knew he was 20 Madison's action made her a hero.
N的“长度”（a,b的长度为b-a）的最小值为（ ）