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Hexdumps used with xxd look something like this:

00000000: 666f 6f20 6261 7220 7370 616d 2065 6767  foo bar spam egg
00000010: 730a                                     s.


Your task is to convert a hexdump in this form in to the number of bytes used.

Rules:


Usual loopholes forbidden.
This is code-golf, so shortest valid answer in bytes wins.
You may or may not include the newline at the end of text (0a). This means that if the hexdump ends in a newline (0a), that input may have it's output reduced by one.
An empty input (literally nothing: empty list/string/etc.) must output 0.
Input can be taken in any form
Input will be valid ASCII with no control characters
The input must contain the whole hexdump


Test cases:

00000000: 4865 6c6c 6f2c 2077 6f72 6c64 2120 4865  Hello, world! He
00000010: 6c6c 6f2c 2077 6f72 6c64 210a            llo, world!.


returns 28 or 27

00000000: 0a                                       .


returns 1 or 0

00000000: 6368 616c 6c65 6e67 650a                 challenge.


returns 10 or 9

00000000: 4865 6c6c 6f2c 2077 6f72 6c64 21         Hello, world!


returns 13




returns 0 (This is literally nothing: empty list/string/etc.)

Explanations would be nice for non-standard languages.
Retina 0.8.2, 8 bytes

.{51}

.


Try it online! Explanation:

.{51}


Delete the first 51 characters of each line. (Lines can only have between 52 and 67 characters, so this always matches once per line.)

.


Count the remaining non-newline characters.

7 bytes if empty input did not have to be supported:

.{52}




Try it online! Explanation:

.{52}



Delete the first 52 characters of each line. (Lines can only have between 52 and 67 characters, so this always matches once per line.)




Count 1 more than the number of remaining characters (including newlines).