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We met President Zelensky in the blacked-out hallways of his command center in Ukraine's capital, Kyiv.

Mr. President, what has it been like working under these conditions?

We found a way how to work. We don't have any other way. You found a way how to work, you don't have any other way.

Weeks of war, attacks on civilians and neighborhoods have exposed a savage Russian strategy,

but still, Zelensky's capital city holds.

No one expected Ukraine to fight Russia to a standstill.

And I wonder, how did you manage?

We united as a nation.

We became a single fist.

On the coast of El Salvador, about an hour from the capital is a town called Elzante.

Black volcanic sand gives way to warm water and an exceptional point break that draws surfers from around the world.

Among them is an American expat with a degree in economics, who locals believe can answer one of the most challenging questions of our time.

What's Bitcoin?

I'm Leslie Stahl.

I'm Bill Whitaker.

I'm Anderson Cooper.

...