The graph of a one-to-one and onto function f defined on the interval [2, 6] is given in the figure.
Given that the area of the shaded region is 13 square units,
$$\int _ { 2 } ^ { 6 } f ^ { - 1 } ( x ) d x$$
What is the value of the integral?\\
A) 18\\
B) 19\\
C) 20\\
D) 21\\
E) 22