Investors knew Berkshire Hathaway was buying up Amazon. Now they know just how much.

Warren Buffett's conglomerate revealed in a filing with the Securities and Exchange Commission Wednesday that it bought 483,300 shares in the first quarter.

With Amazon's closing price Wednesday of $1,871.15 a share, that puts the value of the stake at just over $904 million. Berkshire Hathaway's position may have changed since the end of the first quarter.

Amazon's stock rose modestly in late Wednesday trading.

Buffett first said earlier this month in an interview with CNBC that Berkshire was buying up Amazon, but that he wasn't doing the purchasing himself.

"One of the fellows in the office that manage money ... bought some Amazon so it will show up in the 13F," he told the network just before the Omaha giant's annual shareholder meeting kicked off.

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