newbie on C#.net

I have created a sample program that will add the 4 numbers and it will divide it to 4. Theres no error on the code but the output is wrong. . . i can't figure out what is worong. So please help meh. This is my code: private void btnCalculateGrade_Click(object sender, System.EventArgs e) { double a; a = double.Parse(this.txt1stgrading.Text) + double.Parse(this.txt2ndgrading.Text) + double.Parse(this.txt3rdgarding.Text) + double.Parse(this.txt4thgrading.Text) / 4; this.lblTotal.Text = a.ToString(); }