$\tan ^{-1}x +\tan ^{-1} y +\tan ^{-1} z= \large\frac{\pi}{2}$$=>1-xy-yz-zx=$

Question

$$\begin {array} {1 1} (1)\;1 & \quad (2)\;0 \\ (3)\;-1 & \quad (4)\;2 \end {array}$$