Let R be the circle centered at (0,0) with radius 10. The lines x = 6 and y = 4 divide R into four regions R1, R2, R3, and R4. Let [Ri] denote the area of region Ri. If
[R1] > [R2] > [R3] > [R4],
then find [R1] - [R2] - [R3] + [R4]