not the best solution and not very clever.

• Dont define methods in one line, as it is not very readable
• Use spaces between operators (+) and parameters to make the code more readable
• Using a variable like sum for the result of the addition makes the code more self explenatory

For anyone wondering, this is definitely and absolutely NOT best practice! Its not even a clever solution.

I think you dont even need to work of this edge case.
"abc"[1...-1] # => "b"
"ac"[1...-1] # => ""
"a"[1...-1] # => ""

Thats the only one that may be a problem
""[1...-1] #=> nil

i think its not a good Ruby practice to use the "return" keyword in the last statement