user: Question: What is the oldest year with a main span feet of 1,640 in South Korea?
Columns: ['Rank', 'Main span metres', 'Main span feet', 'Year opened', 'Country']
Types: ['real', 'text', 'text', 'real', 'text']
Sample row: [1.0, '1,991', '6,532', 1998.0, 'Japan']
SQL:

assistant: SELECT MIN("Year opened") FROM "Table" WHERE "Main span feet" = '1,640' AND "Country" = 'South Korea';