What does it take to start a company from scratch and build it into a $2 billion business that trades on the New York Stock Exchange? Ask James Park, who did just that. Nine years ago, he founded Fitbit ”FIT” , which is now one of the leading makers of wearable fitness trackers.

The path getting there wasn’t easy, with many “zig-zags” according to Park. He dropped out of Harvard when he was 21 to the dismay of his Korean immigrant parents. They wanted their son to be a doctor. But Park tells Fortune’s Susie Gharib that he took the risk because he wanted to “do something to leave my own mark on the world.”

Fitbit’s wristbands track everything from heart rate to steps walked to hours slept in one day. Today, millions of people wear the trackers, and Fitbit has turned out to be a phenomenal success story.

Park shares the lessons he learned along the way and has some advice for would-be entrepreneurs just starting out. “Raise more capital than you think you need, “ he says, adding, “that cushion of cash lets you sleep at night.”