choosesubject c# switch

using system;
class choosesubject
{
    static void main()
    {
        int i;
        string str;
        console.writeline ("please choose your favorite subjects:-1 is quit.");
        console.writeline ("1.chinese.");
        console.writeline ("2.maths.");
        console.writeline ("3.english.");
        console.writeline ("4.physical.");
        console.writeline ("-1 is quit.");
        do
        {
            str=console.readline ();
            i=int32.parse (str);
            switch(i)
            {
                case 1:
                    console.writeline ("you like chinese very much.");
                    break;
                case 2:
                    console.writeline ("you like maths very much.");
                    break;
                case 3:
                    console.writeline ("you like english very much.");
                    break;
                case 4:
                    console.writeline ("you like physical very much.");
                    break;
                case -1:
                    break;
                default:
                    console.writeline ("i'm sorry.you can't choose this subject.");
                    break;
            }

        }while(i!=-1);
        console.writeline ("byebye!");
    }
}