  We constrain the circum-burst medium profile with the rise behavior of the
very early afterglow light curves of gamma-ray bursts (GRBs). Using this
method, we find a constant and low-density medium profile for GRB 060418 and
GRB 060607A, which is consistent with the inference from the late afterglow
data. In addition, we show that the absence of the IR flashes in these two GRB
afterglows is consistent with the standard external reverse shock model, which
thus renders models like the highly magnetized GRB outflow being unnecessary to
a certain extent.
