The founder and CEO of Facebook owns over 400 million shares of the company, meaning stock fluctuations hit him the hardest. The trick is figuring out exactly how hard — and that’s where things get a little difficult.

As of April 14, 2017, the company’s last proxy statement, Zuckerberg owned over 2.6 million shares of Class A stock and nearly 411 million Class B shares. In September, though, he announced plans to sell as many as 75 million shares over the following 18 months “to fund the philanthropic initiatives of [he] and his wife, Priscilla Chan,” according to a filing.

So, for argument’s sake, let’s say he’s halfway through that sales goal (unlikely, but it doesn’t hurt to be conservative) — bringing his total holdings to approximately 377 million shares.

Given the company’s 4.5% drop on Friday, that would mean Zuckerberg lost more than $3.1 billion, on paper at least. (If he hasn’t sold any of the 75 million shares he’s planning to, the loss escalates to nearly $3.5 billion.)

Of course, Facebook shares will almost certainly rebound. And analysts say they expect the changes will drive higher ad prices and could result in more money for Facebook, something that always cheers investors.

Ultimately, though, Zuckerberg’s likely not concerned. He’s already pledged to give away 99% of his net worth in his lifetime.