How to calculate totals inside gridview?

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C#

I have a gridview(Gridview1) that has 5 columns as below:
1. The 1st column is databound. It binds to a “CustomerID” column of SQL Server customers table;
2. The 2nd column is databound. It binds to a “CustomerName” column of SQL Server customers table;
3. The 3rd column is databound. It binds to a “ProductID” column of SQL Server customers table;
4. The 4th column is an ItemTamplate. It binds to a “UnitPrice” column of SQL Server customers table.This column contains TexBox (unitPriceTextBox) control inside ItemTamplate;
5. The 5th column is an ItemTamplate. It binds to a “DiscountPercentage” column of SQL Server customers table.This column contains TexBox (discountPercentageTextBox) control inside ItemTamplate;
6. The 6th column is an ItemTamplate. It does not bind to any SQL Server table. But has to contain the total value of the row. This column contains TexBox (totalWithVTATextBox) control inside ItemTamplate;
ALL I NEED TO NOW IS HOW TO CALCULATE THE TOTAL UNIT PRICE IN A FOOTER LABEL (unitPriceTotalLabel) WHEN THE PAGE 1st LOADS AND EVEN WHEN THE USER MAKE CHANGES TO A COLUMN CELL LIKE CHANGING UNIT PRICE OR DELETING A SPECIFIC ROW.
I ALSO WANT TO KNOW HOW TO CALCULATE THE TOTAL IN EACH ROW, THIS IS: THE UNIT PRICE WITH VTA.
I appreciate any help.