SUM a specific column in next rows until a condition is true

Here is a table of articlesI want to store sum of Mass Column from next rows in sumNext Column based on a condition.

If next row has same floor (in floorNo column) as current row, then add the mass of next rows until the floor is changed

E.g : Rows three has sumNext = 2. That is computed by adding the mass from row fourrow five because both rows has same floor number as row three.

Here is the query, that is generating this table, I just want to add sumNext column with the right value inside.

WITH items AS (SELECT 
SP.id,
    SP.mass,
    SP.symbol,
    SP.floorNo
FROM articles SP
        ORDER BY  
          DECODE(SP.symbol,
            'P',1,
            'D',2,
            'L',3,
              4 ) asc)
SELECT CLS.*
FROM items CLS;