Before going to Iberia, Napoleon decided to address several lingering issues with the Russians. At the Congress of Erfurt in October 1808, Napoleon hoped to keep Russia on his side during the upcoming struggle in Spain and during any potential conflict against Austria. The two sides reached an agreement, the Erfurt Convention, that called upon Britain to cease its war against France, that recognized the Russian conquest of Finland from Sweden, and that affirmed Russian support for France in a possible war against Austria "to the best of its ability." Napoleon then returned to France and prepared for war. The Grande Armee, under the Emperor's personal command, rapidly crossed the Ebro River in November 1808 and inflicted a series of crushing defeats against the Spanish forces. After clearing the last Spanish force guarding the capital at Somosierra, Napoleon entered Madrid on 4 December with 80,000 troops. He then unleashed his soldiers against Moore and the British forces. The British were swiftly driven to the coast, and they withdrew from Spain entirely after a last stand at the Battle of Corunna in January 1809.
The Erfurt Convention recognized the Russian takeover of which formerly Swedish possession? (If the question is unanswerable, say "unanswerable")
Finland