Bob has to run some errands around town. Look at the map below to calculate his distance if he starts at home, goes to the video store, then the grocery store, grabs a fast snack, and ends at home. How many units did he walk? Coordinate grid with points labeled home, video store, fast snacks, and grocery store. Home is located at negative 4, 1, video store is at 2, 1, fast snacks is at negative 4, negative 2, and the grocery store is at 2, negative 2.

A: 15 units
B: 18 units
C: 20 units
D: 36 units