It looks like basketball fans are more excited about March Madness than usual this year.

The National Collegiate Athletic Association tournament begins this week as 68 teams compete for the chance to win the championship; the final two teams will play for the title on April 3. Americans are expected to wager $10.4 billion on the tournament this year, up from $9.2 billion in 2016. That’s roughly equivalent to the gross domestic product of several countries including Armenia, Namibia and Malta.

The American Gaming Association (AGA), a national trade group that represents the casino industry, estimates each year how much Americans will bet and says that the increase has happened as online betting has surged. Just $295 million, or 3% of that betting, will be legal, the AGA says. Betting on sports, including in casual office pools that offer prizes to winners, is illegal; betting in the state of Nevada through sports books is one of the few legal channels to do so.

Because of the many combinations of teams that could advance, March Madness betting is notoriously difficult — so difficult, in fact, that Warren Buffett has said he will give $1 million every year for life to any Berkshire Hathaway employee who picks this year’s Sweet 16 correctly. “That’s not Warren Buffett being a generous guy,” Wall Street Journal columnist Jason Gay wrote. “That’s Warren Buffett telling everyone this March Madness stuff is really stupid hard!”