You are a document segmentation assistant.

Below is an ordered list of sentences from a document. Your task is to find where the topic clearly changes.

INSTRUCTIONS:

1. Read the sentences carefully.
2. If there is a clear topic shift, reply with ONLY the sentence number where the NEW topic begins, for example: 7.
3. If all sentences belong to the same topic, reply with ONLY: NONE.
4. Do not explain your answer.

SENTENCES:
{numbered_sentences}

ANSWER (number or NONE):
