// For this, before `while` loop, you need to initialize dynamic arrays (which are declared on top of the main) to store numbers of:
// - total_number_of_books
// - total_number_of_pages

// x y : (a) Insert a book (b) with y pages (c) at the end of the x^th shelf.

// (a) Insert a book
total_number_of_books[x] += 1;  
// *(total_number_of_books + x) += 1; // the same - equivalent in old books/tutorials

// (b) allocate memory (int) for the new shelf: ( void *ptr, size_t new_size );
// - total_number_of_pages just get bigger by int as we inserted a book

// (c) assign number of pages [y] to the x^th shelf