The alphabetical series is R(18), M(13), I(9), F(6), D(4), C(3). Subtract 5, then subtract 4, then subtract 3, then subtract 2, then subtract 1. After M(18-5=13), subtracting 4 gives I(9). After F(6), subtracting 2 gives D(4), then subtracting 1 gives C(3). The blanks are I and C.