user: Question: When Middle Prut Valley is the land formation what is the highest of which currently forests, km² ?
Columns: ['land formation', 'area, km²', 'of which currently forests, km²', '% forests', 'habitat type']
Types: ['text', 'real', 'real', 'text', 'text']
Sample row: ['Northern Moldavian Hills', 4630.0, 476.0, '10.3%', 'forest steppe']
SQL:

assistant: SELECT MAX("of which currently forests, km²") FROM "Table" WHERE "land formation" = 'Middle Prut Valley';