Let $f: \mathbb{R} \rightarrow \mathbb{R}$ be a non-negative continuous function, and let
$$
\mu([a, b)):=\int_a^b f(x) \mathrm{d} x, \quad-\infty<a \leqslant b<+\infty .
$$
Prove that this formula defines a Lebesgue-Stieltjes measure on the real line. ( $f$ is called the density of such a measure.) Write $F$.