The calculated distance (air line) between Changyŏn and Sangsŏng-ni is approximately 115 miles respectively 186 kilometers.
By car or train, the actual journey to Sangsŏng-ni is certainly longer, as only the direct route (as the crow flies) between Changyŏn and Sangsŏng-ni has been calculated here.
Air line (approximately)
186 kilometers
100 nautical miles