Eight cities A, B, C, D, E, F, G and H are connected with one-way roads \(R_1\), \(R_2\), \(R_3\), \(R_4\), \(R_5\) and \(R_6\) in the following manner:
- \(R_1\) leads from A to C via B;
- \(R_2\) leads from C to D and then via B to F;
- \(R_3\) leads from D to A and then via E to H;
- \(R_4\) leads from F to B via G;
- \(R_5\) leads from G to D; and
- \(R_6\) leads from F to H.
The minimum number of road segments that have to be blocked in order to make all traffic from B to D impossible is: