Database Change Sizes

SET SERVEROUTPUT ON;
DECLARE
   --Cursor Declaration
   CURSOR c1
   IS
        SELECT SUM (ROUND ( (tablespace_usedsize * 8 * 1024) / 1024 / 1024, 2))
                  AS "used space",
               snap_id
          FROM DBA_HIST_TBSPC_SPACE_USAGE
         WHERE snap_id IN (SELECT snap_id
                             FROM dba_hist_snapshot
                            WHERE TO_CHAR (BEGIN_INTERVAL_TIME) BETWEEN '16/10/2011%18%'
                                                                    AND '20/10/2011%18')
      GROUP BY snap_id
      ORDER BY snap_id ASC;
   --Variable Declaretion
   row_counter      NUMBER;
   db_used_size_1   NUMBER;
   db_used_size_2   NUMBER;
   snap_time        TIMESTAMP;
   total_change     NUMBER;
BEGIN
   row_counter := 0;
   total_change := 0;
   db_used_size_2 := 0;
   db_used_size_1 := 0;
   FOR emp_rec IN c1
   LOOP
      row_counter := row_counter + 1;
      IF MOD (row_counter, 2) = 0
      THEN
         db_used_size_2 := emp_rec."used space";
      ELSE
         db_used_size_1 := emp_rec."used space";
      END IF;
      IF row_counter != 1
      THEN
         EXECUTE IMMEDIATE 'select begin_interval_time from dba_hist_snapshot where snap_id =' || emp_rec.snap_id INTO snap_time;
         IF     db_used_size_2 - db_used_size_1 != 0
            AND db_used_size_1 != 0
            AND db_used_size_2 != 0
         THEN
            IF MOD (row_counter, 2) != 0
            THEN
               DBMS_OUTPUT.put_line (
                     SUBSTR (TO_CHAR (snap_time),
                             1,
                             INSTR (snap_time, ',') - 1)
                  || ' -> '
                  || TO_CHAR (db_used_size_1 - db_used_size_2)
                  || 'MB'
                  || '--> %'
                  || TO_CHAR (
                        ROUND (
                             100
                           * (db_used_size_1 - db_used_size_2)
                           / db_used_size_1,
                           2)));
               total_change :=
                  total_change + (db_used_size_1 - db_used_size_2);
            ELSE
               DBMS_OUTPUT.put_line (
                     SUBSTR (TO_CHAR (snap_time),
                             1,
                             INSTR (snap_time, ',') - 1)
                  || ' -> '
                  || TO_CHAR (db_used_size_2 - db_used_size_1)
                  || 'MB'
                  || '--> %'
                  || TO_CHAR (
                        ROUND (
                             100
                           * (db_used_size_2 - db_used_size_1)
                           / db_used_size_1,
                           2)));
               total_change :=
                  total_change + (db_used_size_2 - db_used_size_1);
            END IF;
         END IF;
      END IF;
   END LOOP;
   DBMS_OUTPUT.put_line ('Total Change ' || TO_CHAR (total_change || ' MB'));
END;