Billionaire investor Warren Buffett told CNBC on Monday he would judge President Donald Trump after four years, first and foremost, by how safe the U.S. has been kept.

“That is the No. 1 of the chief executive of the United States,” he said. “And that’s not an easy job.”

Buffett said on “Squawk Box” his top concern is how to prevent rogue nations from getting weapons of mass destruction, and he singled out North Korea as a specific threat.

The health of the economy at the end of four years is a second yardstick by which Buffett said he’d evaluate the Republican Trump administration.

“And then third, I’ll judge him on if the economy does well, which I expected it to do, how wide the participation in a better economy extends.”

The chairman and CEO of Berkshire Hathaway, who had supported Democrat Hillary Clinton for president, said he would have judged her by the same three standards.

Reminding investors of his past statements, Buffett pointed out that he said the U.S. economy would be fine under Clinton or Trump.

To that end, Buffett told CNBC Monday that mixing politics and investment strategies would be a “big mistake.”

“Probably half the time [in] my adult life, I’ve had a president other than the one I voted for,” he said. “But that’s never taken me out of stocks.”

Concerning Trump’s cabinet picks, Buffett praised Secretary of State Rex Tillerson, former chairman and CEO of Exxon, as “the kind of person” he would have chosen.

“Tillerson makes a lot of sense,” he said. “Tillerson is going to be working for the United States in that job.”

“I don’t worry at all about whether somebody comes from the oil industry or if they have a lot of money,” Buffett said.