FOR løkke med runtime input

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

namespace WindowsFormsApp8
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnKnap_Click(object sender, EventArgs e)
        {
            int start = int.Parse(txtStart.Text);
            int slut = int.Parse(txtSlut.Text);

            int sum = 0;
            for (int i = start; i <= slut; i++)
            {
                sum = sum + i;
            }
            lblSvar.Text = sum.ToString();
        }
    }
}