599. 12 км 956 м + 8 км 87 м

To solve this problem, we need to add the lengths given. First, we add the meters: 956 m + 87 m = 1043 m. Since 1043 m is equal to 1 kilometer and 43 meters, we carry over 1 km to the kilometer sum. Then, we add the kilometers: 12 km + 8 km + 1 km (carried over) = 21 km. The remaining meters are 43 m. The total length is 21 km and 43 m.