CAT 2022 Linear and Quadratic Equations — Previous Year Question

Let $a, b, c$ be non-zero real numbers such that $b^2 \lt 4 a c$ , and $f(x)=a x^2+b x+c$ . If the set $S$ consists of al integers $m$ such that $f(m)\lt0$ , then the set $S$ must necessarily be