C# Celsius F

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 upr3c
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (textBox1.Text != "")
                calc();
        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {
            if (textBox1.Text != "")
                calc();
        }
        void calc()
        {
            double p, r;
            if (comboBox1.Text == "F")
            {
                p = double.Parse(textBox1.Text);
                r=(p-32)*5/9;
                textBox2.Text = r.ToString();
            }
            if (comboBox1.Text == "C")
            {
                p = double.Parse(textBox1.Text);
                r = p*9/5+32;
                textBox2.Text = r.ToString();
            }
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            comboBox1.Items.Add("F");
            comboBox1.Items.Add("C");
            comboBox1.SelectedIndex = 0;
        }
    }
}