9000 = 9 * 1000 = (10 - 1) * 10 ** 3 900 = 9 * 100 = (10 - 1) * 10 ** 2 90 = 9 * 10 = (10 - 1) * 10 ** 1 + 9 = 9 * 1 = (10 - 1) * 10 ** 0 ----- 9999 = 9 * 1111 = (10 - 1) * 1111 9999 + 1 ----- 10000 = 10 ** 4