Read this: The Arena Football League (AFL) is the highest level of professional indoor American football in the United States. It was founded in 1987 by Jim Foster, making it the third longest-running professional football league in North America, after the Canadian Football League and the National Football League. It is played indoors on a 68-yard field (about half the distance of an NFL field), resulting in a faster-paced and higher-scoring game. The sport was invented in the early 1980s and patented by Foster, a former executive of the United States Football League and the National Football League.

Where does the Arena Football League rank amongst professional football leagues in North America by longevity?
What is the answer? (If it cannot be answered, return "unanswerable")
third