Glad to help =)

Yes, not sure why is there, thanks

strip() is not necesary here

I'm running into the exactly same issue using python

Test Results:
Basic tests
Small numbers (5 Passed, 0 Failed)
Bigger numbers
Log
123456789
123456798
✔ Test Passed
Log
1234567890
1234567908
✔ Test Passed
Log
9876543210
-1
✔ Test Passed
Log
9999999999
-1
✔ Test Passed
Log
59884848459853
59884848453589
✘ 59884848453589 should equal 59884848483559 

While my code is producing 59884848453598, I can provide further details such as the code itself.. Anyway Looks like the test is wrong since the input number is not there.. Not sure how this could have passed for other people doing the python kata

Very elegant!