int a = Convert.ToInt32(TextBoxTUNA.Text);
Problem in above code with TextBoxTUNA.Text value 2181003000 is
You can not store big no in small.
-2,147,483,648 .. 2,147,483,647 is the range of int datatype; so how can you store 2181003000 in int!!
Its like try to put 1.5 liter into 1 liter can.
You can do something like
Int32 a = Convert.ToInt32(TextBoxTUNA.Text);