What is the least common element of all carcinogenic molecules?
label = '+' mean molecules are carcinogenic
SELECT T.element FROM ( SELECT T2.element, COUNT(DISTINCT T2.molecule_id) FROM molecule AS T1 INNER JOIN atom AS T2 ON T1.molecule_id = T2.molecule_id WHERE T1.label = '+' GROUP BY T2.element ORDER BY COUNT(DISTINCT T2.molecule_id) LIMIT 1 ) t