Sets And Relations

Let the relation R on the set $\mathrm{M}=\{1,2,3, \ldots, 16\}$ be given by $\mathrm{R}=\{(x, y): 4 y=5 x-3, x, y \in \mathrm{M}\}$.

Then the minimum number of elements required to be added in R , in order to make the relation symmetric, is equal to