Piecewise linear function from a graph

For positive real numbers $S$ and $K$, the function $H_K(S)$ is defined as $H_K(S) = \max(S - K, 0)$, where $\max(a, b) = a$ when $a > b$ and $b$ when $a \le b$. The graph below shows the plot of a function $N(S)$ versus $S$ (zero for $S \le 10$, rising linearly with slope 1 from $(10, 0)$ to $(20, 10)$, and constant at 10 for $S \ge 20$). $N(S)$ can be expressed as ________.