Reducing of materials

Dear,

Can someone help with good elegant excel formula which can reduce total stock for specific material in unsorted list where materials are being repeated in unsorted way so stock will be reduced for that value and for all last values for that material, e.g.:

(3 columns: 1. materials: A, B, C...; 2. values; 3. result )

A 30 total stock for material A - 30 = ?
B 30 total stock for material B - 30 = ?
C 60 total stock for material C - 60
D 50 total stock for material D - 50
A 10 total stock for material A - 10 - 30 (30 as first value for mat.A)
B 20 total stock for material B - 20 - 30
C 50 total stock for material C - 50 - 60
A 40 total stock for material A - 40 - 10 - 30
B 20 total stock for material B - 20 - 20 - 30
.....
........
...........
For example for mat. 1182929 total stock need to be reduced for every value for that material.
Total is 5973 , but need to be reduced for every qty of mat. 1182929

Thanks in advance,

Regards,

C.