Declare
    total station.id%type;
    without_duplicate station.id%type;
    CURSOR st IS
    SELECT count(city),count(distinct city) from station;
BEGIN
    OPEN st;
    loop
        FETCH st into total,without_duplicate;
        EXIT WHEN st%NOTFOUND;
        DBMS_OUTPUT.PUT_LINE(total-without_duplicate);
    END loop;
    CLOSE st;
END;
/
SET SERVEROUTPUT ON;
/