Before revision:  In passing, we have rederived and made more precise the result of [   P. L. Krapivsky and S. Redner, Am.
After revision:  In passing, we have rederived and made more precise the result of [ L. Turban, J. Phys. A 25, 127 (1992) P. L. Krapivsky and S. Redner, Am.