Auto Filling of textbox based on dropdown in infopath 2010

Hi all

I am having two control.

  1.Dropdown 2.Textbox

In dropdown i have loaded some value from sharepoint list,based on dropdown value i need to fill textbox value automatically in infopath designer 2010.

Example:

List

Field1   Field2

 1         aaaa

 2         bbbb

if my dropdown value is 1 means,then aaa should fill automatically in textbox