Hello everyone
In my practice (out of contest) I can solve Upto Div2_D problems, but when I face problems that I need math, numbertheory or counting methods skills to solve it, suddenly I stop and can't solve it without reading tutorial or I can't solve it even after reading the tutorial!
I'm asking if you can help me by giving me books, websites or anything that can help me in my dilemma.
Thank you in advance =D