若直线 $l_1:\left\{\begin{array}{c}x=1-2t,\\ y=2+\mathit{kt}.\end{array}\right.\left(t\mathit{为参数}\right)$ 与直线 $l_2:\left\{\begin{array}{c}x=s,\\ y=1-2s.\end{array}\right.\left(s\mathit{为参数}\right)$ 垂直，则 $k=$       ．