1. In isosceles trapezoid $A B C D$, $C D=2 A B=40, E$ is the point on $A D$ closer to $A$ that divides $A D$ into three equal segments, $F G$ is parallel to $A D$. The trapezoid $A B C D$ is divided into four regions, and the areas of these four regions $S_{1}, S_{2}, S_{3}$, $S_{4}$ increase by the same amount successively, then the length of line segment $G H$ is $\qquad$ —.
$18$