Not an error - both 92 and 94 are missing from the sequence, so the sequence is invalid and the answer is -1

After reading this specific thread, I finally understand the problem! Thank you, might try this one soon, but not sure how I'd approach it...hmmm...

"Take the example: 2-1-3-4..." The Author should have used this example instead of 1234 example. Thanks for the explanation, wanted to skip this one because i did not understand the challange, but now i will try it.

hi goatmilk... Always consider with 2 base rules:

1. It is all about your point of view
2. Lower buildings hide behind higher buildings (1 hides behind 2)

Take the example: 2-1-3-4.
From left to right (point of view), you can see 3 buildings because 1 hides behind the 2 but 3 does not hide behind the 2 and 4 does not hide behing the 3.
From right to left, you can see only 1 building. The 4 hides all other buildings because they are lower.

Does my explanation help you?