Replace the question mark with a number

After waiting for what seemed like an eternity, the stewardess announces over the intercom that "we're just waiting for the pilots."
The passengers look out the window and see two men, dressed as pilots walking towards the plane. Both men are using guide dogs and appear to be blind. There are murmurs among the passengers, and some believe it is a joke.
The men board the plane and go into the cockpit. More concerned murmurs and uneasy chuckles from the passengers. The plane taxis normally to the runway and begins it's takeoff. As passengers look out the window they realize they are nearing the end of the runway. The entire passenger cabin begins screaming but the plane lifts off just before the end of the runway. The passengers calm down and chuckle to themselves, at this point believing that they fell for a joke.
In the cockpit, the pilot turns to his copilot and says "you know, one day those people are gonna scream too late and we're all gonna die!"