What is the expiation for a vow that cannot be fulfilled, yet has become obligatory to keep?

Allah Almighty described His righteous servants as fulfilling their vows, and the Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah Almighty, let him obey Him; and whoever vows to disobey Allah Almighty, let him not disobey Him." You must fulfill your vow unless it involves disobedience or is impossible, in which case you are required to offer an expiation for breaking an oath. The expiation for breaking an oath is: feeding ten poor persons, or clothing them, or freeing a slave; and if you cannot find these, then fasting for three days.