I love math. Just after the high school entrance ceremony, I meet a beautiful girl: Milka. Milka is a mathematical genius. She gives me many math problems, she shows me many elegant solutions. Milka and I spend a long time by discussing math in the school library.
One year later, I meet another mathematical girl: Tetra. Tetra is one year younger than me, and asks me to teach her math. While I teach her, she begins to understand math and to love its elegance gradually.
The second series is about Pythagoras Theorem, Elementary Number Theory, Group, Ring, Field, and Fermat's Last Theorem.