2. Let $A(1,-1,0), B(2,0,1)$ be points on the skew lines $a, b$, respectively, and the direction vectors of lines $a, b$ are $\boldsymbol{a}=(1,0,-1), \boldsymbol{b}=(1,-1,1)$, then the distance between the two skew lines $a, b$ is $\qquad$ .