Travel to all Points

There are N special points on the 2-D plane. You can travel from a special point (xi, yi) to another special point (xj, yj) if and only if |xi - xj| + |yi - yj| ≥ D. You need to find the largest integer D, such that we can start at some special point and visit each of the other special points, directly or through other special points. Note that you can visit a special point more than once. In