Longest balanced entry exit stretch

A ward's door sensor writes one character per event: 'I' when someone enters, 'O' when someone leaves. For a staffing report, find the length of the longest contiguous stretch of events containing exactly as many entries as exits. Return 0 if no such stretch exists. The input contains only 'I' and 'O' characters and may be empty. Example: events = "IOOI" gives 4, since the whole log has two entries and two exits.