The Brexit is nearly upon us, and Bookmaker.eu has set the odds: -350 that the UK stays in the European Union and +250 that it leaves.

If you’re not a betting person, those numbers probably mean nothing to you. But take note, because betting odds have proven in the past to be more accurate in predicting political decisions than both polling and expert opinion.

Here’s how the odds work: “-350” means that for every $350 a person bets, he gets $100 in profit. “+250” means that for every $100 a person bets, he gets $250 in profit.

A negative sign (in this case -350) means that bookmakers favor that event happening. A positive sign (in this case +250) means that bookmakers consider it the underdog choice. In other words, you have to bet more to make money on the favored event than you do to make money on the underdog.

If there is a chance that something may or may not happen, you can bet that someone is betting on it (except for rocket launches, people can’t bet on that yet). Bookmaker.eu has also recently set odds on political events from who will be Trump’s vice presidential nominee to whether Scotland would gain independence.

Betting on the Brexit, however, is already the biggest political betting event in British history, The Guardian reports. More than £40.5 million (around $60 million) had bet on the Brexit as of Monday. And bookies have maintained that the UK staying in Europe is the more likely event since bets started being placed in February.

Want to put some dough in, but don’t know what the Brexit is? John Oliver has a scathing explainer. Remember to follow the odds though, because bookies have a vested interest and a long track record of being correct.