cout << "Hello world!" << endl;
   double number;
    start:
    cout<< "please choose your number:"; cin >>number;
    if (number==19||number==29||number==39||number==59||number==79||number==89)
    {
        cout << "worng number" << endl;
    }
    else if (number<10)
{
        cout << "number is too small" << endl;
    }
    else if (number>100)
{
        cout << "number is too large" << endl;
    }

    else{cout <<"your number:" << number <<endl;}

    goto start; // loop back
//--------------------------------------------------------end of the block
     return 0;