To Pass ID from the one table to another table

<pre lang="cs">con.Open();
SqlCommand cmd = new SqlCommand("insert into Exe_ContactDetails values(@Name,@Mobile,@Address,@UserId)", con);
cmd.CommandType = CommandType.Text;
string str = "Select ID from Team_Registration where UserName= '" + Session["New"] + "'";
//int N = Convert.ToInt32(str);
cmd.Parameters.AddWithValue("@Name", txtName.Text.ToString());
cmd.Parameters.AddWithValue("@Mobile", txtMobile.Text.ToString());
cmd.Parameters.AddWithValue("@Address", txtAddress.Text.ToString());
cmd.Parameters.AddWithValue("@UserId", str);
cmd.ExecuteNonQuery();
con.Close();</pre>

when I executed this code (Conversion failed when converting the nvarchar value 'Select ID from Team_Registration where UserName= 'madhu'' to data type int.)getting this error.
Please suggest to me how can I errase this error.

How can I store the Team_registration ID in another table column UserId

Please give me a answer.....