145.24 $\star \star$ Find the maximum value of the following expression:
$$
|\cdots||| x_{1}-x_{2}\left|-x_{3}\right|-x_{4}\left|-\cdots-x_{1990}\right|,
$$

where $x_{1}, x_{2}, \cdots, x_{1990}$ are distinct natural numbers from 1 to 1990.
Notice that, when $x \geqslant 0, y \geqslant 0$, we have $|x-y| \leqslant \max \{x, y\}$, and $\max \{\max \{x, y\}, z\}=\max \{x, y, z\}$. Therefore, when $n=1,2, \cdots, 1990$, we have $|\cdots|\left|x_{1}-x_{2}\right|-x_{3} \left|-\cdots-x_{n}\right| \leqslant \max \left\{x_{1}, x_{2}, \cdots, x_{n}\right\}$. Thus, under the given conditions, the given expression does not exceed $\max \left\{x_{1}, \cdots, x_{1990}\right\}=1990$. However, the expression cannot equal 1990, because the parity (odd or even nature) of the value of the expression must be the same as the parity of the sum $x_{1}+x_{2}+\cdots+x_{1990}=1+2+\cdots+1990=995 \times 1991$, which is odd. However, the following example shows that the value of the expression can reach 1989: $|\cdots||| 2-4|-5|-$ $3|-\cdots-(4 k+2)|-(4 k+4)|-(4 k+5)|-(4 k+3)|-\cdots-1986|-1988 \mid-$ $1989|-1987|-1990|-1|=1989$. In summary, the maximum value sought is 1989.