user: Question: Opponent of at San Francisco 49ers had what lowest week?
Columns: ['Week', 'Date', 'Opponent', 'Result', 'Venue', 'Attendance']
Types: ['real', 'text', 'text', 'text', 'text', 'real']
Sample row: [1.0, 'September 29, 1957', 'Chicago Bears', 'W 21-17', 'City Stadium', 32132.0]
SQL:

assistant: SELECT MIN("Week") FROM "Table" WHERE "Opponent" = 'at San Francisco 49ers';