If in the expansion of \( (1 + x)^{p} (1 - x)^{q} \), the coefficients of \( x \) and \( x^{2} \) are \( 1 \) and \( -2 \), respectively, then \( p^{2} + q^{2} \) is equal to:
\( 20 \)
\( 8 \)
\( 18 \)
\( 13 \)