Hello, in this blog I will explain how to set a Div2E/F on Codeforces in $$$2$$$ simple steps.

- Create an easy problem
- Add the condition $$$\gcd = k$$$

Advantages of this method:

- You don't need to think about the details of the solution. It's enough to solve for all gcds and use either Mobius function or inclusion-exclusion.
- On the other side, the problem is suitable for Div2E/F because most contestants are too lazy to study basic number theory.
- This method has been used successfully in a lot of recent rounds! Examples: 1436F - Sum Over Subsets, 1554E - You, 1559E - Mocha and Stars

I hope this blog will be useful for problemsetters!