user: Question: Tell me the total number of crest length for year construction larger than 1957 for mattmark
Columns: ['Name', 'Canton', 'Height (meters)', 'Crest length (meters)', 'Type', 'Year of construction', 'Name of the Lake']
Types: ['text', 'text', 'real', 'real', 'text', 'real', 'text']
Sample row: ['Grande Dixence', 'Valais', 285.0, 695.0, 'Gravity', 1961.0, 'Lac des Dix']
SQL:

assistant: SELECT SUM("Crest length (meters)") FROM "Table" WHERE "Year of construction" > 1957.0 AND "Name" = 'mattmark';