What is the expected number of random binary walk hits zero within $K$ steps

Question

I am looking for a closed form equation for the expected number of times symmetric binary walk hits zero value in K steps (at the limit of large number of steps).

In other words:

$x[0]=0$

$x[k]=x[k-1]+b[k]$,

where $b[k]$ is $-1$ or $1$ (probability $0.5$ each).

What is the expected value of the number of times $x[i]==0$ (where i=0 to K), ie number of zero-value hitting in the first K steps.

(A Reference to a textbook or paper would be nice)

Thanks!