Calculate the number 2420

A man is in bed with his wife when there is a knock at the door. He rolls over and looks at his clock, and it's 3:30 in the morning. "I'm not getting out of bed at this time," he thinks, and rolls over.
Then a louder knock follows. "Aren't you going to answer that?" says his wife. So he drags himself out of bed and goes downstairs. He opens the door and there is a man standing on the porch. It didn't take the homeowner long to realize the man was drunk.
"Hi there," slurs the stranger. "Can you give me a push??"
"No, get lost! It's half past three. I was in bed," says the man and he slams the door. He goes back up to bed and tells his wife what happened and she says, "That wasn't very nice of you. Remember that night we broke down in the pouring rain on the way to pick the kids up from the babysitter and you had to knock on that man's house to get us started again? What would have happened if he'd told us to get lost?"
"But the guy was drunk," says the husband. "It doesn't matter," says the wife. "He needs our help and it would be the Christian thing to help him."
So the husband gets out of bed again, gets dressed, and goes downstairs. He opens the front door, and not being able to see the stranger anywhere he shouts, "Hey, do you still want a push?" And he hears a voice cry out, "Yeah, please." So, still being unable to see the stranger he shouts, "Where are you?"

And the drunk replies, "Over here, on the swing."