Validation Parse System

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 assignment_5_1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button_exit_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void button_calculate_Click(object sender, EventArgs e)
        {

            bool mph_ok = false;
            bool hour_ok = false;
            string tb_input_mph = textBox_mph.Text;
            string tb_input_hours = textBox_hours.Text;
            int tb_input_mph_int;
            int tb_input_hours_int;

            if (int.TryParse(tb_input_mph, out tb_input_mph_int))
            {
                mph_ok = true;
            }

            if (int.TryParse(tb_input_hours, out tb_input_hours_int))
            {
                hour_ok = true;
            }

            if (mph_ok && hour_ok)
            {

                listBox_output.Items.Clear();

                for (int count = 1; count <= tb_input_hours_int; count++)
                {
                    listBox_output.Items.Add("After hour " + count + " the distance is " + (count * tb_input_mph_int));
                }

            }
            else
            {
                MessageBox.Show("Please enter a number for MPH and hours.");
                textBox_mph.Focus();
            }

        }
    }
}