5. Given complex numbers $z_{1}, z_{2}, z_{3}$ such that $\frac{z_{1}}{z_{2}}$ is a pure imaginary number, $\left|z_{1}\right|=\left|z_{2}\right|=1,\left|z_{1}+z_{2}+z_{3}\right|=1$, then the minimum value of $\left|z_{3}\right|$ is
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