user: Question: Which First elected is the lowest one that has an Incumbent of William Stewart?
Columns: ['District', 'Incumbent', 'Party', 'First elected', 'Result']
Types: ['text', 'text', 'text', 'real', 'text']
Sample row: ['Pennsylvania 1', 'Thomas B. Florence', 'Democratic', 1848.0, 'Retired Democratic Hold']
SQL:

assistant: SELECT MIN("First elected") FROM "Table" WHERE "Incumbent" = 'William Stewart';