Better calculator in c#

using System;
using System.Windows.Forms;

namespace CalculatorThatIsBetter
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        decimal firstNumber=0, secondNumber=0, thirdNumber=0;
        byte counter = 0;


        private void buttonCompute_Click(object sender, EventArgs e)
        {
            if(counter == 0) {
                if (decimal.TryParse(textBoxInput.Text, out firstNumber)==false)
                {
                    ShowError_Parse(); } counter++; labelFirstNum.Text = firstNumber.ToString("N2");
                textBoxInput.Text = "";
            }
            else if(counter == 1) {
                if (decimal.TryParse(textBoxInput.Text, out secondNumber) == false)
                {
                    ShowError_Parse(); } counter++; labelSecondNum.Text = secondNumber.ToString("N2");
                textBoxInput.Text = "";
            }
            else if (counter > 1)
            {
                // do math
                if (labelOperator.Text=="+") { thirdNumber = firstNumber + secondNumber; }
                if (labelOperator.Text == "-") { thirdNumber = firstNumber - secondNumber; }
                if(labelOperator.Text == "*") { thirdNumber = firstNumber * secondNumber; }
                if (labelOperator.Text == "/"&&secondNumber!=0) { thirdNumber = firstNumber / secondNumber; }
                if (labelOperator.Text=="/"&&secondNumber==0) { MessageBox.Show("You cannot divide by zero","Error"); }
                MessageBox.Show("Result: "+thirdNumber,"Your Answer");
                counter = 0;
            }

        }

        private void buttonAdd_Click(object sender, EventArgs e)
        {
            labelOperator.Text = "+";
        }

        private void buttonMultiply_Click(object sender, EventArgs e)
        {
            labelOperator.Text = "*";
        }

        private void buttonSubtract_Click(object sender, EventArgs e)
        {
            labelOperator.Text = "-";
        }

        private void buttonDivide_Click(object sender, EventArgs e)
        {
            labelOperator.Text = "/";
        }

        private void button1_Click(object sender, EventArgs e)
        {
            textBoxInput.Text += "1";
        }

        private void button2_Click(object sender, EventArgs e)
        {
            textBoxInput.Text += "2";
        }

        private void button3_Click(object sender, EventArgs e)
        {
            textBoxInput.Text += "3";
        }

        private void button4_Click(object sender, EventArgs e)
        {
            textBoxInput.Text += "4";
        }

        private void button5_Click(object sender, EventArgs e)
        {
            textBoxInput.Text += "5";
        }

        private void button6_Click(object sender, EventArgs e)
        {
            textBoxInput.Text += "6";
        }

        private void button7_Click(object sender, EventArgs e)
        {
            textBoxInput.Text += "7";
        }

        private void button8_Click(object sender, EventArgs e)
        {
            textBoxInput.Text += "8";
        }

        private void button9_Click(object sender, EventArgs e)
        {
            textBoxInput.Text += "9";
        }

        private void buttonDecimal_Click(object sender, EventArgs e)
        {
            textBoxInput.Text += ".";
        }

        private void buttonClear_Click(object sender, EventArgs e)
        {
            counter = 0;
            firstNumber = 0;
            secondNumber = 0;
            thirdNumber = 0;
            labelFirstNum.Text = firstNumber.ToString("N2");
            labelOperator.Text = "+";
            labelSecondNum.Text = secondNumber.ToString("N2");
        }

        private void button10_Click(object sender, EventArgs e)
        {
            textBoxInput.Text += "0";
        }
        public void ShowError_Parse()
        {
            MessageBox.Show("Failed to parse","Error");
        }
    }
}