Lebesgue measure of the euclidean ball

Question

The following is written in "An introduction to Geometric Measure theory" by F. Maggi.

Let $B$ be an open euclidean ball in $\mathbb{R}^n$ and $\omega_n=|B|$, where $|B|$ is Lebesgue measure of $B$, then it is easily seen that $\omega_1=2$

I do not easily see this, it seems like I overthink it and miss something obvious.

Answer 1

$\omega_1$ is the Lebesgue measure of the open unit ball in $\mathbb{R}$. The open unit ball in $\mathbb{R}$ is just $(-1,1)$, which clearly has Lebesgue measure $2$ (it is just the length of the interval).