public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        int s1, s2;
        private void Form1_Load(object sender, EventArgs e)
        {
            s1 = 10;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            s2 = 20;
            MessageBox.Show((s1+s2).ToString());
        }