Warren Buffett, Saudi Arabia and the Wall Street party of the year: WHAT YOU NEED TO KNOW TODAY

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There is a lot going on this morning, so here's a quick roundup.

• Berkshire Hathaway owned $1 billion worth of Apple shares as of the end of the first quarter. Appaloosa Management, the hedge fund led by billionaire David Tepper, sold out of Apple, in contrast.
• Buffett is in the bidding war for Yahoo. Here's what he thinks about Yahoo's internet business and CEO Marissa Mayer.
• Moody's downgraded Saudi Arabia, saying: 'The buffers will continue to erode.'
• Goldman Sachs says oil's biggest problem could be over— and now crude is on a charge. This chart shows why Goldman Sachs decided to do its U-turn on oil.

In other news, Business Insider's Linette Lopez went to the biggest Wall Street party of the year, and everyone was completely miserable.

Nasdaq CEO Bob Greifeld talked to Business Insider about David-and-Goliath battles, making computers work harder, and the future of trading.

Famous short seller Carson Block spoke to Business Insider's Bob Bryan about making the market a better place.

And hedge fund billionaire Jim Chanos just called out his entire industry for failing to deliver the one thing that matters.

On Wall Street, a bunch of junior bankers pretended to be startup founders for a day. UBS is investing in a tech startup, and is looking to do more deals. And one anecdote sums up Wall Street's hatred for a Fed rule.

Elsewhere, the warning signs of a credit slowdown are beginning to appear, and the middle class is vanishing in America's cities. 

And lastly, this chart shows the age when most people feel the least happy. So you've got that to look forward to. 

Here are the top Wall Street headlines at midday:

Pfizer just struck a $5.2 billion deal for an eczema drugmaker ― Pfizer is planning to buy biopharmaceutical company Anacor.

The Chinese taxi company that got $1 billion from Apple may be eyeing a US IPO - Ride-hailing service Didi Chuxing, Uber Technologies Inc's main rival in China, is working toward an initial public offering in the United States.

BANK OF AMERICA: We're hitting 'peak youth' and there's a 'demographic tidal wave' coming ― "We are living in the midst of the most remarkable demographic transition in history," according to Bank of America Merrill Lynch.

Something funky is happening between oil and the dollar ― Historically, there's been a pretty consistent correlation between the commodity's price and the currency.

This guy saw his Microsoft coworkers struggle to manage their money, and it gave him the idea for a $150 million app ― Jon Xu and the rest of his soon-to-be cofounders were leading project teams at Microsoft when they first noticed a problem.

MORGAN STANLEY: Things are only going to get worse for Russia's battered economy ―Morgan Stanley has a warning for Russia: don't expect things to get better any time soon.

US airlines are terrified of this European upstart you've never heard of — they should be ― Norwegian Air International's expansion into the US has ruffled feathers. It's a potentially industry-changing move that US airlines and unions have vigorously opposed.

Boeing's most important plane could be headed for trouble ― The 787 Dreamliner is the first new plane Boeing has designed and built from scratch in 20 years, but its entry into service has been far from smooth.

Ferrari has done something no one thought was possible with the new 488 GTB supercar ― Ferrari builds three types of cars: sports cars, Grand Touring or "GT" cars, and of course rare and exotic hypercars, such as the current LaFerrari, which sells for more than a million bucks.