According to the international paper size standarts
a sheet of A0 paper is equal to 1m².
Successive paper sizes in the series A1, A2, A3, and so forth,
are defined by halving the preceding paper size.
Each of these notebooks have the same area (1m²)
with different paper sizes.
(32 paged A5, 64 paged A6, 128 paged A7 notebook)