Microsoft co-founder Bill Gates knows a lot of things. The price of pizza rolls is not one of them.

The tech icon and philanthropist appeared on Ellen Wednesday and played a game called "Bill's Grocery Bills," where Gates had to correctly guess the prices of some commonly used household items.

When asked by Ellen Degeneres the last time he went to the supermarket: "a long time ago."

The goal was to guess 3 out of the 5 products within $1 of the price, and the audience got a prize.

The good news: Gates must be a regular flosser because his first guess ($4) on dental floss was off by a few cents.

However, we're not sure where Gates gets his pizza rolls because he thought a bag of 120 rolls cost $22.

Fortunately for the audience, after several guesses, they eventually got their prizes.