smc

For any polynomial, even a polynomial with more than one variable, the sum of all the coefficients (including the constant, which is the coefficient of $x^0{y}^0$) is found by setting all variables equal to $1$. Note that we don't have to worry about whether a constant is a coefficient of an "invisible" $x^0{y}^0$ term, because there is no such term here. Setting $x=y=1$ gives $(-1{)}^{18}$, which is equal to $1$, which is answer $\boxed{1}$.