The figure shows two vectors drawn on a grid: $\vec a$ ends at $(4, 1)$ and $\vec b$ ends at $(-1, 3)$. What is $\vec a\cdot\vec b$?

Fig. 1: A graph on a square grid with x from -3 to 6 and y from -1 to 4. A blue arrow a runs from the origin to the point (4, 1). A green arrow b runs from the origin to the point (-1, 3).