uniswap v2: flashloan fee

Question

I know that there is a 0.3% fee in the flashloan of Uniswap v2, which is deducted from the transaction. When I use the swap () function for flashloans, in the end I should pay back the money. I need to calculate the fee, I see someone calculating it this way, and I cannot understand:

uint256 _ fee = (_amount * 3) / 997 + 1

This calculation is somewhat similar to the getAmountIn(), but not the same.

Answer 1

The formula and explanation can be found here in the Uniswap docs.