Calculator C# program simple one have a issue

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace WindowsApplication1
{
    public partial class Form1 : Form
    {
        int a,b,c;
    

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {

            a = int.Parse(textBox1.Text);
            b = int.Parse(textBox2.Text);
            c = a + b;
                   
            label.Text = c.ToString();
        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {

        }

        private void button2_Click(object sender, EventArgs e)
        {

            a = int.Parse(textBox1.Text);
            b = int.Parse(textBox2.Text);
            c = a - b;

            label3.Text = c.ToString();

        }
    }
}