editing DataGrid cells and binding some cells to each other

hi all

am using visual studio 2012 WPF c# .

on a DataGrid i have with 3 columns.the three columns are empty.i know how to enter data into the datagrid.but what i dont know is when i enter the data in first and second column for a specific entity i want the data in the third column to be the sum of the first two columns.

i am new to wpf.

thanks in advance