6. 85 Find the maximum value of the following expression:
$$|\cdots|\left|x_{1}-x_{2}\right|-x_{3}\left|-\cdots-x_{1990}\right|,$$

where $x_{1}, x_{2}, \cdots, x_{1990}$ are distinct natural numbers from 1 to 1990.
[Solution] When $x \geqslant 0, y \geqslant 0$, it is obvious that $|x-y| \leqslant \max \{x, y\}$ 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, the expression given in the problem does not exceed
$$\max \left\{x_{1}, \cdots, x_{1990}\right\}=1990$$

However, the expression cannot equal 1990, because the parity of the value of the expression should be the same as the parity of the sum
$$\begin{aligned}
& x_{1}+x_{2}+\cdots+x_{1990}=1+2+\cdots+1990 \\
= & \frac{1990 \cdot 1991}{2}=995 \cdot 1991
\end{aligned}$$

which is odd. The following specific example shows that the value of the expression can reach
$$\begin{array}{l}
1989:|\cdots||| 2-4|-5|-3|-\cdots-(4 k+2)|-(4 k+4)|-|(4 k \\
+5)|-(4 k+3)|-\cdots-1986|-1988|-1989|-1987|-1990 \mid-1 \\
\mid=1989 .
\end{array}$$

In conclusion, the maximum value sought is 1989.