Quadrilateral A B C D with side lengths A B=7, B C=24, C D=20, D A=15 is inscribed in a circle. The area interior to the circle but exterior to the quadrilateral can be written in the form \dfrac{a \pi-b}{c}, where a, b, and c are positive integers such that a and c have no common prime factor. What is a+\stackrel{c}{b}+c ?
Answer Choices
A. 260
B. 855
C. 1235
D. 1565
E. 1997