Let the relation \( R \) on the set \( M = \{1, 2, 3, \ldots, 16\} \) be given by \( R = \{(x, y) : 4y = 5x - 3 , \; x , y \in M\} \). Then the minimum number of elements required to be added in \( R \), in order to make the relation symmetric, is equal to:
