I am sorry, it's like 5 years back when I did it, and I don't remember (nor I am inclined in pursuing it further). (I didn't gave up because it became impossibl

I am sorry, it's like 5 years back when I did it, and I don't remember (nor I am inclined in pursuing it further). (I didn't gave up because it became impossible, I just became less interested.) But lot of those harder ones - as far as I can tell - required pretty deep knowledge of number theory, which even though I studied math (10 years ago), didn't have. I think it would be nice if there was sometimes a hint about what the people should study to be able to solve it.