Graph Theory - How can I calculate the number of vertices and edges, if given this example An algorithm book Algorithm Design Manual has given an description:

Consider a graph that represents the street map of Manhattan in New York City. Every junction of two streets will be a vertex of the graph. Neighboring junctions are connected by edges. How big is this graph? Manhattan is basically a grid of 15 avenues each crossing roughly 200 streets. This gives us about 3,000 vertices and 6,000 edges, since each vertex neighbors four other vertices and each edge is shared between two vertices.

If it says "The graph is a grid of 15 avenues each crossing roughly 200 streets", how can I calculate the number of vertices and edges? Although the description above has given the answers, but I just can't understand.
Can anyone explain the calculation more easily?
Thanks

This is so simple, I mean 15 avenues crossing 200 streets, which means there are 15 * 200 = 3000 crossings, i.e. 3000 nodes. 
each nodes have upper, lower, left and right neighbors, so for each node, there are 4 edges connecting to the neighbors. However, each edge has been counted twice since node 1 has an edge connected with node 2, and the node 2 has the same edge connected to node 1
So that is totally 3000*4/2 = 6000 edges
