Given that x+y+z=0, and ((x+y)/2)=z-1, find the value of z.
已知x+y+z=0，且((x+y)/2)=z-1，求z的值．
<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<result>2/3 = around 0.666667</result>