The mystery of the lost book

Oskar loves numbers and counting, but he doesn't enjoy reading. That's why in his new book, he prefers to look for page numbers, which he adds and subtracts in various ways. But then his book mysteriously disappears, and Oskar, along with his grandfather, sets off to find it. The search gets really complicated when the book reappears with a mysterious cipher…
Will Oskar manage to crack the code and find out what really happened? And will you be able to crack the cipher in this book? Get to it!