Phineas is driving toward an intersection at a rate of 30 km/h while along the perpendicular road through the intersection Ferb drives away from the intersection at a rate of 50 km/h. When Phineas was already 4 km from the intersection, Ferb was already 3 km from it. At this exact instant, how fast is the distance between the two changing? Are they getting closer to or getting farther from each other at this instant?