Hi, I'm trying to use the erlang C formula in excel to solve for the number of staff needed if i give the probability of wait time... can someone help me re-write the equation?

ORIGINAL ERLANG C m= number of staff, u = traffic intensity, p = \frac {u}{m}

E_{c}(m,u) = \frac {Poission (m,u,false)}{Poission(m,u,false) + (1-p) * Poission (m-1,u,true)}

if everything else is given, how would i solve for m?