Replace the question mark with a number

A man had just settled into his seat next to the window on the plane when another man sat down in the aisle seat, and put his black Labrador Retriever in the middle seat between them.
The first man looked very quizzically at the dog and asked why it was allowed on the plane.
The second man explained that he was a DEA agent, and that the dog was a "drug-sniffing dog."
He went on, "His name is Sniffer and he's the best there is.
I'll show you once we get airborne, when I put him to work."
The plane took off, and once it leveled out, the agent said "Watch this." He told Sniffer to "search".
Sniffer jumped down, walked along the aisle, and finally sat very purposefully next to a woman for several seconds.
Sniffer then returned to his seat and put one paw on the agent's arm.
The agent said, "Good boy!", turned to t he man and said, "That woman is in possession of marijuana, so I'm making a note of her seat number and the authorities will apprehend her when we land."
"Say, that's pretty neat," replied the first man.
Once again, the agent sent Sniffer to search the aisles. The Lab sniffed about, sat down beside a man for a few seconds, returned to his seat, and placed two paws on the agent's arm.
The agent said, "That man is carrying cocaine, so again, I'm making a note of his seat number for the police."
"I like it!" said his seat mate.
The agent then told Sniffer to "search" again.
Sniffer walked up and down the aisles for a little while, sat down for a moment, and then came racing back to the agent, jumped into the middle seat and pooped all over the place.
The first man was really grossed out by this behavior and couldn't figure out how or why a well-trained dog would act like that. He asked the agent, "What's going on?"
The agent nervously replied, "He just found a bomb!"