The calculated distance (air line) between Sangsŏng-ni and Hŭngnam is approximately 61 miles respectively 98 kilometers.
By car or train, the actual journey to Hŭngnam is certainly longer, as only the direct route (as the crow flies) between Sangsŏng-ni and Hŭngnam has been calculated here.
Air line (approximately)
98 kilometers
53 nautical miles