14. (15 points) In the arithmetic expression shown in the figure, 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. Find the sum of the digits $a, b, c, d$.

将上面的文本翻译成英文，请保留源文本的换行和格式，直接输出翻译结果。
【Analysis】According to the vertical calculation, the result of a four-digit number minus a three-digit number is 2. From $998+2=1000, 999+2=1001$, we can deduce that the result of $a 6 b+4 c d$ is 1000 or 1001, and then solve it according to the method of addition.

【Solution】According to the vertical calculation, we get: $a 6 b+4 c d=1000$ or $a 6 b+4 c d=1001$;
(1) When $c=3$, $b+d$ carries over,

Assume $a 6 b+4 c d=1000$;
In the units place, $b+d=10$, carry 1 to the tens place;
In the tens place: $6+c+1=10, c=3$, carry 1 to the hundreds place;
In the hundreds place: $a+4+1=10, a=5$;
Thus, $a+b+c+d=5+10+3=18$;
Assume $a 6 b+4 c d=1001$;
In the units place, $b+d=11$, carry 1 to the tens place;
In the tens place: $6+c+1=10, c=3$, carry 1 to the hundreds place;
In the hundreds place: $a+4+1=10, a=5$;
Thus, $a+b+c+d=5+11+3=19$.
(4) When $c=4$, $b+d$ does not carry over,

At this point, $b+d=1, 0+1=1$ meets the requirement.
$$
a+b+c+d=5+1+4=10 .
$$

Answer: The sum of the digits $a, b, c, d$ is $10, 18$ or 19.