A one-sided sign test $p$-value from a tiny sample

Let $(X_i,Y_i), i=1,\dots,5$ be the bivariate pair of observations and $Z_i=X_i-Y_i, i=1,\dots,5$. For a given sample the number of $Z_i>0$ is 2. Hence the $p$-value to test the null hypothesis that $H_0:\text{median}(Z)=0$ against $H_1:\text{median}(Z)<0$ is: