$2]$ Add the count

Suppose you have a text file like this:

$3$

Alice in Chains

Yellow Submarine

The show must go on

Where the first number indicates the number of lines $($can be a different number$)$

Make a program to read the file for read, close he file, count the letters, open the file again to append, and write a message saying how many letters it has, so the file will end like this:

$3$

Alice in Chains

Yellow Submarine

The show must go on

The lines have $50$ letters.