The calculated distance (air line) between Seongnam-si and Naesŏ is approximately 170 miles respectively 273 kilometers.
By car or train, the actual journey to Naesŏ is certainly longer, as only the direct route (as the crow flies) between Seongnam-si and Naesŏ has been calculated here.
Air line (approximately)
273 kilometers
147 nautical miles