cumulating by adding previous sales to daily sales

I had excel-02 version in that my file contains three columns with Column A(customer) column B(Daily Sales) Column C(Cumulative Sales).
In column A list of 100 customers appears.
In column B I enter daily sales.
In column C I take previous day cumulative sales figure and to that todays daily sales in order to arrive todays cumulative sales.
This is cumbersome to do for 100 customers.
Please help me so that whenever i enter daily sales figure it will give cumulative sales automatically.