Riverland

There is a country with a large river system. The cities in this country are numbered $1$ through $N$; city number $1$ is the capital. The river system forms a rooted tree such that the capital is the root. Each river connects two cities and flows towards the root. For each non-root city $i$, there is a river that flows from this city to city $p_i$. There is also a port in each city. In the count