Bill Gates has revealed his "one big regret" in a stream of tweets giving advice to new university graduates around the world.
The Microsoft co-founder told his 35 million followers that "looking back on when I left college, there are some things I wish I had known".
His greatest regret was that "when I left school, I knew little about the world’s worst inequities," the multi-billionaire said. "Took me decades to learn."
Mr Gates added: "AI, energy, and biosciences are promising fields where you can make a huge impact. It's what I would do if starting out today.
"You know more than I did when I was your age. You can start fighting inequity, whether down the street or around the world, sooner.
"Meanwhile, surround yourself with people who challenge you, teach you, and push you to be your best self. As @MelindaGates does for me."
He also said "the world is getting better", citing cognitive scientist Steven Pinker and his book, The Better Angels Of Our Nature.
Mr Gates added: "Sounds crazy, but it’s true. This is the most peaceful time in human history.
"That matters because if you think the world is getting better, you want to spread the progress to more people and places.
"It doesn’t mean you ignore the serious problems we face. It just means you believe they can be solved.
"This is the core of my worldview. It sustains me in tough times and is the reason I love my work. I think it can do same for you."
Earlier this year Mr Gates was confirmed as the world's richest man for the fourth year running, with his estimated wealth totalling some $86 billion (£67 billion).
