If n is 1, then "This kata is very interesting!"

Take every 2nd char from the string, then the other chars, that are not every 2nd char, and concat them as new String."

Becomes "hskt svr neetn!Ti aai eyitrsig" the lenght is always the same, you only took every 2nd char, and forgot to concatenate the other part.

Print the input, it seems in that case it's empty and your code returned 0

All numbers in the list are positive numbers and the list can be empty.

Print the input values, see why.

read the complete error message to see from where the error is raised, then amend your code. ;) (and don't forget to print stuff to the console if needed at some point).

EDIT : and read the message just below...?

Closing then.

Use markdown formatting when posting code so indentation is kept. That code doesn't work for inputs like this:

s1 = "a"
s2 = "aaa"

Your code only checks if the first letter of s2 is in s1 or not.