5.3.2.20 Various problems in the space

Condition: In a regular pyramid \( S A B C D \) the side edge forms an angle with the plane of the base equal to \( 45^{\circ} \). At the height \( S O \) of the pyramid, a point \( K \) is taken - the middle of \( S O \). Find the angle formed by the straight line \( D K \) with the plane \( S A D \).