5.3.2.24 Various Problems in the Space

Condition: at the base of the pyramid \ (s a b c d \) lies a square with a side \ (a \), and the lateral rib \ (s b \) perpendicular to the plane of the base and equal to the side of the base. On the ribs \ (s d \) and \ (a d \) the points \ (m \) and \ (l \) are taken respectively - the middle of these ribs. Find the distance between straight \ (m l \) and \ (a c \).