CREATE TABLE table_55849 ("Presidency" FLOAT, "President" TEXT, "Date of Birth" TEXT, "Took office" FLOAT, "Left office" TEXT)

What is the Took Office Date of the Presidency that Left Office Incumbent?
SELECT SUM("Took office") FROM table_55849 WHERE "Left office" = 'incumbent'
SELECT SUM("Took office") FROM "table_55849" WHERE "Left office" = 'incumbent'