Buckley purchased a collectible stamp for $600. He was thrilled to learn that the stamp price had moved up, so one year later he purchased another stamp for $700. At the end of the second year, he sold one of the stamps for $800. He held his original stamp for the third year. Buckley then sold his stamp at the end of the fourth year for $1,000. What was Buckley's dollar-weighted rate of return?