The calculated distance (air line) between Hwaseong-si and Naesŏ is approximately 165 miles respectively 266 kilometers.
By car or train, the actual journey to Naesŏ is certainly longer, as only the direct route (as the crow flies) between Hwaseong-si and Naesŏ has been calculated here.
Air line (approximately)
266 kilometers
143 nautical miles