What is the missing number?

A Blonde Guy walks into a bar very down on himself. As he walks up to the bar the bartender asks, "what's the matter?"

The Blonde Guy replies, "well I've got these two horses (sniff, sniff), and well… I can't tell them apart. I don't know if I'm mixing up riding times or even feeding them the right foods."

The bartender, feeling sorry for the guy, tries to think of something he can do.

"Why don't you try shaving the tail of one of the horses?"

The man stops crying and says, "that sounds like a good idea, I think I'll try it."

A few months later he comes back to the bar in worse condition than he was before.

"What's the matter now?" the bartender asks.

The Blonde Guy, in no condition to be in public, answers, "I shaved the tail of one of the horses (sob, sob), but it grew back and I can't tell them apart again!"

The bartender, now just wanting him to shut up or leave says, "why don't you try shaving the mane, maybe that will not grow back."

He stops crying, has a few drinks, and leaves.

A few months later the guy is back in the bar.

The bartender has never seen anybody in this sorry of a state.

Without the bartender even asking the fellow breaks into his problems. "I.. I shaved the (sob) mane of one of the (sniff) horses, and… it… it… grew back!"

The bartender, now furious at the guy's general stupidity, yells, "for crying out loud, just measure the stupid horses. Perhaps one is slightly taller than the other one!"

The guy cannot believe what the bartender has said and storms out of the bar.

The next day the Blonde Guy comes running back into the bar as if he had just won the lottery.

"It worked, it worked!" he exclaims.

"I measured the horses and the black one is two inches taller than the white one!"