A bank vault has three locks with a different key for each lock. Each key is owned by a different person. To open the door, at least two people must insert their keys into the assigned locks. The signal lines $A,B,$ and $C$ are $1$ if there is a key inserted into lock $1,2,$ or $3,$ respectively. Write an equation for the variable $Z$ which is $1$ iff the door should open.