public void Calc_Sum(int A, int B, int C)
        {
            Double dSum = 0;
            textBox3.Text = "";
            for (int i = A; i <= B; i += C)
            {
                dSum += i;
                textBox3.AppendText(i + " + ");
            }
            textBox3.Text = textBox3.Text.Substring(0, textBox3.TextLength - 2);
            textBox3.AppendText(" = " + dSum);
        }

        private void button1_Click(object sender, EventArgs e)
        {
            int iStart, iEnd;
            Double dSum = 0;

            iStart = int.Parse(textBox1.Text);
            iEnd = int.Parse(textBox2.Text);

            if (iStart % 2 == 0)
                iStart++;

            Calc_Sum(iStart, iEnd, 2);
        }

        private void button2_Click(object sender, EventArgs e)
        {
            int iStart, iEnd;
            Double dSum = 0;

            iStart = int.Parse(textBox1.Text);
            iEnd = int.Parse(textBox2.Text);

            if (iStart % 2 == 1)
                iStart++;

            Calc_Sum(iStart, iEnd, 2);
        }

        private void button3_Click(object sender, EventArgs e)
        {
            int iStart, iEnd;
            Double dSum = 0;

            iStart = int.Parse(textBox1.Text);
            iEnd = int.Parse(textBox2.Text);
            Calc_Sum(iStart, iEnd, 1);
        }