why is the focal length negetive in concave mirror and positive in convex mirror

in mirrors and lens there is a convention for calculation of distances from the pole/center of the mirror/lens.

1. we measure all the distances from the pole/center of the mirror/lens.

2. if we go in the direction of the light rays incoming we take it to be positive and if we go opposite we take it to be negative.

So, since in convex mirror for going towards focus from yhe pole we need to move in backward direction so it is negative.