Congruence condition for minus one to be a quadratic residue

==Statement

Suppose $\text{[math]}$ is an odd prime. Then, $\text{[math]}$ is a quadratic residue modulo $\text{[math]}$ if and only if $\text{[math]}$ . Equivalently, $\text{[math]}$ is a quadratic nonresidue modulo $\text{[math]}$ if and only if $\text{[math]}$ .