If \( \vec{i} + \vec{j} - \vec{k}, -\vec{i} + 2\vec{j} + \vec{k}, \vec{j} + 2\vec{k}, 2\vec{i} - \vec{j} + 2\vec{k} \) are the position vectors of four points \( A, B, C, D \) respectively, then the shortest distance between the lines \( AB \) and \( CD \) is: