The calculated distance (air line) between Slavuta and Starokostyantyniv is approximately 40 miles respectively 65 kilometers.
By car or train, the actual journey to Starokostyantyniv is certainly longer, as only the direct route (as the crow flies) between Slavuta and Starokostyantyniv has been calculated here.
Air line (approximately)
65 kilometers
35 nautical miles