1. Given the set $S=\left\{x \in \mathbf{R} \mid x^{2}+2 x+a=0\right\}$, if $-1 \in S$, then the real number $a=$
$$
(-1)^{2}+2 \cdot(-1)+a=0 \Rightarrow a=1
$$

The translation is as follows:
$$
(-1)^{2}+2 \cdot(-1)+a=0 \Rightarrow a=1
$$