Problems with try, catch blocks.

Having a problem with exception handling. I created a form with 5 textboxes. The first four are for the user to enter the coordinates of two points and the fifth displays the length of a line between the two when clicking button1.

This code compiles.

private void button1_Click(object sender, EventArgs e)
        {
           
                string startingX;
                string startingY;
                string endingX;
                string endingY;
                double line;
                string length;

                startingX = textBox1x.Text;
                startingY = textBox1y.Text;
                endingX = textBox2x.Text;
                endingY = textBox2y.Text;


           
                    double startX = double.Parse(startingX);
                    double startY = double.Parse(startingY);
                    double endX = double.Parse(endingX);
                    double endY = double.Parse(endingY);
              

                line = Math.Sqrt(
                    (endX - startX) * (endX - startX) + (endY - startY) * (endY - startY));

                length = line.ToString();
                textBoxline.Text = length;



When I try to handle exceptions with try, catch in the code below this gives errors on the line = Math.Sqrt operation saying that my end and start variables do not exist in the current context. I tried wrapping all four parse lines in a single block and got the same errors. I just can't seem to get my mind around exceptions. Any help would be greatly appreciated.


private void button1_Click(object sender, EventArgs e)
        {
           
                string startingX;
                string startingY;
                string endingX;
                string endingY;
                double line;
                string length;

                startingX = textBox1x.Text;
                startingY = textBox1y.Text;
                endingX = textBox2x.Text;
                endingY = textBox2y.Text;


            try
            {
                    double startX = double.Parse(startingX);
            }
            catch (FormatException)
            {
                MessageBox.Show("Please enter a valid integer for the first points X coordinate!");
            }
            try
            {
                    double startY = double.Parse(startingY);
            }
            catch (FormatException)
            {
                MessageBox.Show("Please enter a valid integer for the first points Y coordinate!");
            }
            try
            {
                double endX = double.Parse(endingX);
            }
            catch (FormatException)
            {
                MessageBox.Show("Please enter a valid integer for the second points X coordinate!");
            }
            try
            {
                double endY = double.Parse(endingY);
            }
            catch (FormatException)
            {
                MessageBox.Show("Please enter a valid integer for the second points Y coordinate!");
            }
               

                line = Math.Sqrt(
                    (endX - startX) * (endX - startX) + (endY - startY) * (endY - startY));  

                length = line.ToString();
                textBoxline.Text = length;