private void button1_Click(object sender, EventArgs e)
        {
            if (comboBox1.Text == "4")
            {
                if (textBox1.Text == "0")
                    MessageBox.Show("Otpornost mora biti veca od nule", "Otpornost", MessageBoxButtons.OK, MessageBoxIcon.Error);

                //  var black = Color.Black;



                var colours = new string[] { "Black", "Brown", "Red", "Orange", "Yellow", "Green", "Blue", "Violet", "Grey", "Silver", "Gold" };
                var factorDictionary = new Dictionary<char, double>() { { 'p', 0.000000000001 }, { 'n', 0.000000001 }, { 'u', 0.000001 }, { 'm', 0.001 }, { 'R', 1 }, { 'K', 1000 }, { 'M', 1000000 }, { 'G', 1000000000 } };
                string res = string.Empty;

                while (string.IsNullOrEmpty(res))
                {
                    //  MessageBox.Show("Enter resistor value (use R, K, M ,G notation for .): ");
                    res = textBox1.Text;
                }

                var lastChar = res.Last();
                var isUnitCorrect = factorDictionary.ContainsKey(lastChar);
                var value = res.Substring(0, res.Length - 1);
                var isValueCorrect = !value.Any(x => !char.IsDigit(x));

                if (isUnitCorrect && isValueCorrect)
                {
                    double mul = factorDictionary[lastChar];
                    double val = double.Parse(value) * mul;
                    int third = 0;

                    if (val < 1)
                    {
                        val *= 100;
                        third = 9;
                    }
                    else if (val < 10)
                    {
                        val *= 10;
                        third = 10;
                    }

                    res = val.ToString();

                    if (res.Count() > 24)
                        MessageBox.Show("Invalid value");

                    else
                    {
                        label15.BackColor = Color.FromName(colours[res[0] - '0']);
                        label16.BackColor = Color.FromName(colours[res[1] - '0']);
                        label17.BackColor = Color.FromName(colours[third != 0 ? third : res.Count() - 2]);
                    }
                    if (comboBox1.Text == "4")
                    {
                        if (comboBox2.Text == "E24 (5%)")
                            label18.BackColor = Color.Gold;
                        if (comboBox2.Text == "E12 (10%)")
                            label18.BackColor = Color.Silver;
                    }
                }
                else
                    MessageBox.Show("Invalid value!");
            }