Automatically fills the column based on the selection of choice column in the sharepoint list 2010

I have one custom list built in sharepoint 2010 which is choice column.

The Datas of the list are

1.ABC

2.CDE

3.DEF

4.DFG

5.VFG

I need only numbers be automatically fills that in column when adding new item in the list.

for e.g: if i select “1.ABC”,I need to fill that column field as only 1,

if i select “2.CDE”,I need to fill that column field as only 2,

if i select “3.DEF”,I need to fill that column field as only 3,

if i select “4.DFG”,I need to fill that column field as only 4,

if i select “5.VFG”,I need to fill that column field as only 5,

How can i achieve my requirement?

Please Help

Thanks in advance.