Microsoft founder and billionaire Bill Gates on Wednesday criticized 2020 White House hopeful Sen. Elizabeth Warren’s (D-MA) wealth tax proposal, stating that it left him doing “a little math about what I have left over.”

“I’ve paid over $10 billion in taxes. I’ve paid more than anyone in taxes,” Gates, who has an estimated networth of $107 billion, said at the New York Times DealBook conference. ”If I had to pay $20 billion, it’s fine.

“But when you say I should pay $100 billion, then I’m starting to do a little math about what I have left over,” he added.

“Sorry, I’m just kidding,” he then quipped.

Warren has vowed, if elected president, to double the top rate of her wealth tax to 6% from 3%, which would tax individuals 3 cents on every dollar over $1 billion in net worth.

DealBook’s Andrew Sorkin asked Gates if he would be open to discussing alternate tax proposals with the Massachusetts Democrat, to which he replied: “I’m not sure how open-minded she is… or that she’d even be willing to sit down with somebody who has large amounts of money.”

Gates then went on to say that he did not want to make “political declarations” about whether he would support President Donald Trump or Warren for president in 2020.

“I do think no matter what policy somebody has in mind, a professional approach is even – as much as I disagree with some of the policy things that are out there, I do think a professional approach to the office – whoever I decide would have the more professional approach in the current situation probably will weigh, is the thing that I will weigh the most,” said Gates. “I hope the more professional candidate is an electable candidate.”

Warren took to Twitter shortly after Gates’s comment, tweeting that she would be happy to meet with Gates to “explain exactly how much you’d pay under my wealth tax”.

I'm always happy to meet with people, even if we have different views. @BillGates, if we get the chance, I'd love to explain exactly how much you'd pay under my wealth tax. (I promise it's not $100 billion.) https://t.co/m6G20hDNaV — Elizabeth Warren (@ewarren) November 7, 2019

“I promise it’s not $100 billion,” she added.