Excel - sum first values found based on another column

in the table below, I have in column A a reference number and in column B a value that is null or positive:

Reference	Volume
074610	100
074610	0
171546	0
171546	200
751714	0
751714	50
751714	0
789723	25

Outside of the table, I would like to sum column B only on the values that matches the first row with a different reference. In my example, it would mean that I only add

• the first row for reference 074610,
• the first row for reference 171546,
• the first row for reference 751747,
• etc.

The formula should give a result: 100 + 0 + 0 + 25 = 125 but all the formulas I tried give the total of column without taking into account the first row for each reference number.

Does anyone has an idea of fomula that could work?

Thank you !