what is the minimum attendance with stadium being cotton bowl
{'header': ['Week', 'Date', 'Opponent', 'Result', 'Stadium', 'Record', 'Attendance'], 'page_title': '1961 Buffalo Bills season', 'section_title': 'Regular season', 'caption': 'Regular season', 'rows': [['1', 'September 10, 1961', 'Denver Broncos', 'L 22–10', 'War Memorial Stadium', '0–1', '16636'], ['2', 'September 17, 1961', 'New York Titans', 'W 41–31', 'War Memorial Stadium', '1–1', '15584'], ['3', 'September 23, 1961', 'Boston Patriots', 'L 23–21', 'War Memorial Stadium', '1–2', '21504'], ['4', 'September 30, 1961', 'San Diego Chargers', 'L 19–11', 'War Memorial Stadium', '1–3', '20742'], ['5', 'October 8, 1961', 'at Houston Oilers', 'W 22–12', 'Jeppesen Stadium', '2–3', '22761'], ['6', 'October 15, 1961', 'Dallas Texans', 'W 27–24', 'War Memorial Stadium', '3–3', '20678'], ['7', 'October 22, 1961', 'at Boston Patriots', 'L 52–21', 'Boston University Field', '3–4', '9398'], ['8', 'October 29, 1961', 'Houston Oilers', 'L 28–16', 'War Memorial Stadium', '3–5', '21237'], ['9', 'November 5, 1961', 'Oakland Raiders', 'L 31–22', 'War Memorial Stadium', '3–6', '17027'], ['10', 'November 12, 1961', 'at Dallas Texans', 'W 30–20', 'Cotton Bowl', '4–6', '15000'], ['11', 'November 19, 1961', 'at Denver Broncos', 'W 23–10', 'Bears Stadium', '5–6', '7645'], ['12', 'November 23, 1961', 'at New York Titans', 'L 21–14', 'Polo Grounds', '5–7', '12023'], ['13', 'December 3, 1961', 'at Oakland Raiders', 'W 26–21', 'Candlestick Park', '6–7', '8011']]}