Algorithms

There is a jail in the desert. A rebellion broke out and a single guard is immobilized. He helplessly watched the prisoners left one by one with different speed in different directions. Later, he freed himself and took a motorbike with an extra seat. Now, he can follow the footprints and pick up one prisoner at a time and bring them back to jail. Each prisoner moves with constant individual speed v_i and left the jail at time t_i. In which order does the guard bring prisoners back in order to minimize the time