A memory device comprises a memory cell region including a first metal pad, and a peripheral circuit region including a second metal pad and vertically connected to the memory cell region by the first metal pad and the second metal pad, wherein the memory cell region includes a first memory area having first memory cells storing N-bit data and a second memory area having second memory cells storing M-bit data, where ‘M’ and ‘N’ are natural numbers and M is greater than N, and the peripheral circuit region includes a controller configured to read data stored in the first memory area using a first read operation, read data stored in the second memory area using a second read operation different from the first read operation, and selectively store data in one of the first memory area and the second memory area based on a frequency of use (FOU) of the data.