The figure below is a map showing a grid of cities and roads connecting certain pairs of cities. There are 6 columns and 3 rows of cities, forming 5 columns and 2 rows of squares. Paula wishes to travel along exactly 21 of those roads, starting at city (top-left) and ending at city (bottom-right), without traveling along any portion of a road more than once. (Paula is allowed to visit a city more than once.) 
How many different routes can Paula take?
0
1
2
3
4
