There are three routes from A to B: A-C-E-B, A-C-D-B and A-C-D-F-B. It should be noted that the information flow of each route should be minimal. Take the third route as an example, A-C 25, C-D 1 1.

It should also be noted that the information flow of the trunk line must be greater than the sum of the branch lines. For example, C-D 1 1 is greater than 6 of D-B plus 4 of D-F-B. Similarly, 25 of A-C must be greater than 17 of the final sum. Of course, this question is the same. So the answer is 17.