This means that $p \equiv 1 \pmod{4}$.
Now, let's use Euler's Criterion, which states that for any integer $a$ and an odd prime $p$, we have:
$$a^{\frac{p-1}{2}} \equiv \left(\frac{a}{p}\right) \pmod{p}$$
where $\left(\frac{a}{p}\right)$ is the Legendre
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