  The reverse shock (RS) model is generally introduced to interpret the optical
afterglows with the rapid rising and decaying, such as the early optical
afterglow of GRB 990123 (which is also called optical flash). In this paper, we
collected 11 gamma-ray bursts (GRBs) early optical afterglows, which have such
signatures of dominant RS emission. Since the temporal slopes of the optical
flashes are determined by both the medium density distribution index $k$ and
the electron spectral index $p$, we apply the RS model of the thin shell case
to the optical flashes and determine the ambient medium of the progenitors. We
find that the $k$ value is in the range of 0 - 1.5. The $k$ value in this paper
is consistent with the result in Yi et al. (2013), where the forward shock (FS)
model was applied to some onset bumps. However, the method adopted in this
paper is only applicable to GRB afterglows with significant sharp rising and
decaying RS emission. Our results indicate that the RS model can also be
applied to confirm the circumburst medium, further implying that GRBs may have
diverse circumburst media.
