1. The coefficient of $x^{3}$ in the algebraic expression $1+x+x(1+x)+x(1+x)^{2}$ $+\cdots+x(1+x)^{1999}$ is $\qquad$
(Answer with a number)
Ni.1.1331334000.
Starting from the 4th term, there is $x^{3}$, the sum of its coefficients is
$$
\begin{array}{l}
C_{2}^{2}+C_{3}^{2}+\cdots+C_{1999}^{2}=C_{2000}^{3} \\
=\frac{2000 \times 1999 \times 1998}{6}=2000 \times 1999 \times 333 \\
=666000 \times(2000-1)=1331334000 .
\end{array}
$$