Rules for sign convention :
1) All distances should be measured from the pole.
2) The distances measured in the direction of incident light, to be taken positive and measured in the opposite direction of incident light to be taken negative.
3) Height of object and height of image are positive if measured upward from the axis and negative if measured downward.
4) For a concave mirror 'f and 'R' are negative and for a convex mirror these are positive.