double points;
    start:
    cout << "points:"; cin >> points;
    if (points<=0) return 0;

    if(points>=0 && points <=39) // -------------------------------------------------start of the block
{
    cout << "your mark is 2.0" << endl;
}
    else if (points >39 && points <=54)
     {
         cout<< "your mark is 3.0"<< endl;
     }
    else if (points >54 && points <=69)
    {
        cout<< "your mark is 3.5" <<endl;
    }
    else if (points >69 && points <=84)
    {
        cout<< "your mark is 4.0" << endl;
    }
    else if (points >84 && points <=98)
    {
        cout<<"your mark is 4.5" << endl;
    }
    else
    {
        cout <<"your mark is 5.0"<<endl;
    }

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