Find when failures started in log stream

You are given an array of log entries sorted by timestamp ascending. Each log entry contains:

• timestamp (monotonic increasing)
• status ∈ { OK , FAIL }

Assume the system behaves as follows:

• At some time T (possibly never), a failure begins.
• Once a failure begins, every log at time ≥ T has status FAIL (i.e., the statuses are a prefix of OK s followed by a suffix of FAIL s).

Task

Return the earliest timestamp (or index) at which failure started (the first FAIL in the array). If there is no failure, return a sentinel value (e.g., null / -1).

Edge cases to consider

• Empty log array
• All logs are OK
• All logs are FAIL

Complexity goal

Better than linear time if possible.