  We consider a directed percolation process on an ${\cal M}$ x ${\cal N}$
rectangular lattice whose vertical edges are directed upward with an occupation
probability y and horizontal edges directed toward the right with occupation
probabilities x and 1 in alternate rows. We deduce a closed-form expression for
the percolation probability P(x,y), the probability that one or more directed
paths connect the lower-left and upper-right corner sites of the lattice. It is
shown that P(x,y) is critical in the aspect ratio $a = {\cal M}/{\cal N}$ at a
value $a_c =[1-y^2-x(1-y)^2]/2y^2$ where P(x,y) is discontinuous, and the
critical exponent of the correlation length for $a < a_c$ is $\nu=2$.
