The State College Spikes baseball stadium's concession stands previously sold hot dogs for $5 each. At that price, when Joey went to watch a baseball game, he bought 2 hot dogs. But now that the stadium has a special night where hot dogs are just $2, Joey has purchased 4 hot dogs. Using the percent change formula for elasticity, what is the value of Joey's price elasticity of demand for hot dogs? (You'll have to use the percent change formula).