Solution
= Solution
The <probability> of a positive delay is the integral of its density:
$$
\mathbb P(S>0)=\int_0^r\frac{ds}{R}=\frac rR.
$$
Adding the separate no-delay event gives
$$
\boxed{\mathbb P(S=0)+\mathbb P(S>0)=\left(1-\frac rR\right)+\frac rR=1.}
$$
This is the <normalization of a probability distribution> with both discrete and continuous parts.