Excel 2007 : Inventory calculation

hello everyone i have been trying to figure this one out for hours. i hope it can be done as it will be greatly beneficial. I have an excel spreadsheet for inventory. Column A has the unique part numbers, column b has the part numbers i gather from a hand scanner and column C has the quantity i have in stock.

Basically i want to scan my bar codes and have them entered into column b1-b500. I need a formula that can count how many times the bar codes in A1-A500 are matched in B1-B500 and then add it to the quantity which is column c so if i have the master bar code in a of 777 listed in A1 but i scanned the bar code 777 in the b column 9 times column c1 would be 9.

hopefully this is possible, i have tried countif and match to no avail.