Let \( w_{1} \) be the point obtained by the rotation of \( z_{1} = 5 + 4i \) about the origin through a right angle in the anticlockwise direction, and \( w_{2} \) be the point obtained by the rotation of \( z_{2} = 3 + 5i \) about the origin through a right angle in the clockwise direction. Then the principal argument of \( w_{1} - w_{2} \) is equal to:
