A, B, C, and D are four natural numbers arranged in ascending order. The number A is the sum of the numbers B, C, and D (3/17), and the sum of the numbers A and B is the sum of the numbers C and D. (5/7), what is the smallest number of Ding?
甲、乙、丙、丁是从小到大排列的四个自然数，甲数是乙、丙、丁三数的和的(3/17)，甲、乙两数的和是丙、丁的和的(5/7)，丁数最小是多少．
<result>18</result>