The shortest distance between the lines
\[
\vec r=\vec a+t\vec b
\]
and
\[
\vec r=\vec c+s\vec d
\]
where
\[
\vec a=\hat i-2\hat j+2\hat k,
\quad
\vec b=3\hat i-2\hat j-2\hat k,
\]
\[
\vec c=6\hat i+2\hat j+2\hat k,
\quad
\vec d=-4\hat i-\hat k,
\]
is