14. Let the complex numbers $z_{1}, z_{2}$ satisfy
$$
\begin{array}{l}
\left|z_{1}\right|=\left|z_{2}\right|=1 \text {, and } z_{1}+z_{2}-1=0 \text {. } \\
\text { Then }\left(z_{1}-z_{2}\right)^{2}=
\end{array}
$$