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

Here is a table of articles and I 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 four and row 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;