how to retain the checkboxvalue checked in gridview with paging

hi
i have gridview with paging enabled, and have checkbox in trmplate field.
dere are 20 records. i am displaying 5 records at once. i have checkbox i am checking record 3 and 4, and going to next page checking record 9, so. on
and when i click button. it is able to retain only the selected value in the checkbox a. but for the last. all other values are lost.

how to retain the values checke in the checkbox in paging

foreach (GridViewRow row in GridView1.Rows)
{
CheckBox checkbox = (CheckBox)row.FindControl("CheckBox1");
if (checkbox.Checked)
{

int employeeID = Convert.ToInt32(GridView1.DataKeys[row.RowIndex].Value);
}
}

this is my code
plz do provide solution how to retain it checked value in the checkbox