14. (15 points) In the equation on the right, the letters $a, b, c, d$ and “ $\square$ ” represent one of the ten digits from 0 to 9, where $a, b, c, d$ represent four different digits in $\square \square \square \square$. Find the sum of the digits represented by $a, b, c, d$.
$$
\begin{array}{r}
\text { a } 5 \text { b } \\
+\quad 4 \quad \text { c d } \\
\hline \square \square \square \\
-\quad \square \square \\
\hline
\end{array}
$$
【Analysis】First, analyze the result of a four-digit number minus a three-digit number being a single-digit number. The cases can be enumerated, and exclusions can be made accordingly.

【Solution】According to the problem:
The four-digit number - the three-digit number $=2$ can only have 2 possibilities, $1000-998=2$ or $1001-999=2$.
Then, it is required that $5+c=9, a+4=9$. Therefore, $a=5, c=4$. So, the result of $b+d$ can be 10 or 11. Thus, the result of $a+b+c+d$ is 19 or 20.

In conclusion, the answer is 19 or 20.