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