The calculated distance (air line) between Seongnam-si and Kaesŏng is approximately 49 miles respectively 78 kilometers.
By car or train, the actual journey to Kaesŏng is certainly longer, as only the direct route (as the crow flies) between Seongnam-si and Kaesŏng has been calculated here.
Air line (approximately)
78 kilometers
42 nautical miles