Solution
= Solution
The conditioning event selects the exponential component: $P(Y\geq0)=1/2$ and $P(Y\geq1)=e^{-1}/2$. Thus <conditional probability> gives
$$
\boxed{P(Y\geq1\mid Y\geq0)=\frac{e^{-1}/2}{1/2}=e^{-1}.}
$$
The atom at $-1$ is excluded by the conditioning event.