استفاده ار دستور case در ++C

Barnamei ke nomerat daneshjiyan ra daryaft karde
    va tedad meghdar arzeshi an ra chap konad 

// Barnamei ke nomerat daneshjiyan ra daryaft karde
//   va tedad meghdar arzeshi an ra chap konad.

#include<iostream.h>
#include<stdio.h>
#include<conio.h>

void main()
{
 textcolor(7);
 textbackground(1);
 clrscr();
 int a,b,c,d,grade,k;
 k=a=b=c=d=0;
 cout<<"\n\n\tfor end insert '-1'\n";
 cout<<"\n\tenter a grade "<<++k<<" : ";
 cin>>grade;
 if(grade>20 || grade<0 && grade!=-1)
 {
  cout<<"\a\n\terror!!!\n\tEntert Other Grade..." ;
  k--;
  getch();
 }
 while(grade!=-1 )
 {
  switch (grade)
  {
   case 20:
   case 19:
   case 18:
    ++a;
    break;
   case 17:
   case 16:
   case 15:
    ++b;
    break;
   case 14:
   case 13:
   case 12:
    ++c;
    break;
   default :
    if(0<=grade && grade<=11)
     ++d;

  }//end case
   cout<<"\n\n\tenter a grade "<<++k<<" : ";
  cin>>grade;
  if(grade>20 || grade<0 && grade!=-1)
 {
  cout<<"\a\n\terror!!!\n\tEntert Other Grade..." ;
  k--;
  getch();
 }
 }//end while
 clrscr();
 cout<<"\n\n\n\tcounter of(18-20): a="<<a<<"\n\n\tcounter of(15-17):b="<<b;
 cout<<"\n\n\tcounter of(12-14):c="<<c<<"\n\n\tconter number of belowe 12: d="<<d;
 getch();
}