The "minimum number of streets to block" directly redirects us to Max Flow Min Cut Theorem It says the maximum flow we send from a source s to sink t is exactly equal to the capacity of th eminimum cut that seperates s from t. We have to divide the graph into two parts one group containing source and the other one containing sink. A capacity of the cut is the sum of capacities of the edges going from the source's set to the sink's set.

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