Suma N primeros números enteros.

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 numero22
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
//Variables
int numero, suma;
private void button2_Click(object sender, EventArgs e)
{
suma = (numero * (numero + 1)) / 2;
textBox2.Text = suma.ToString();
}

private void textBox1_TextChanged(object sender, EventArgs e)
{
numero = int.Parse(textBox1.Text);
}




private void button1_Click(object sender, EventArgs e)
{
Application.Exit();
}
}
}