Consider an electron with mass m and energy E moving along the x-axis towards a finite step potential of height $U_0$, as shown in the figure. In region $1 (x < 0)$, the momentum of the electron is $p_1 = \sqrt{2mE}$. The reflection coefficient at the barrier is given by $R = (\frac{p_1-p_2}{p_1+p_2})^2$, where $p_2$ is the momentum in region 2. If, in the limit $E>> U_0$, R, then the integer n is ________.