I was working as a problem setter last year on different websites. One day I encountered very interesting math problem.
I couldn't solve it for hours. I gave up, looked up for solution, and liked it so much that I couldn't help but modify it a little bit then introduce to CP community.
So here is the problem I came up with. With the help of Birzhan, my close friend, I submitted it to HackerRank which responded very positively and asked more for similar problems. :)
I definitely suggest you to check out also the editorial.