Generalizing is so difficult

There is a $$0$$ in each cell of a $$20\times 20$$ square grid. You are permitted to perform the following step as many times as you want:

Choose a row or a column, and replace the numbers in it with the numbers $$1, 2, 3, \dots, 20$$ in an order of your choosing.

What is the maximum possible sum of the numbers in the square?

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