how to get multiple datakeys in gridview using button click

Question

 I have a gridview with 2 datakeysname ie date and day.

This is my code :

 

foreach (GridViewRow row in gvCalenderData.Rows)

{

int date =Convert.ToInt32(gvCalenderData.DataKeys[row.RowIndex].Values[0].Tostring);

Session["cal_date"] = date;

string day = gvCalenderData.DataKeys[row.RowIndex].Values[1].ToString();

Session["cal_day"] = day;

} 

 

 but i got this type of error '' Input string was not in a correct format. ''