c program for simple queue

#include<conio.h>
#include<stdio.h>
int f=-1,r=-1,q[20],n=5;
void insert()
{
if(r>=n-1)
{
printf("\n over flow on insert:");
}
else
{
if(r==-1)
{
r=0;
f=0;
}
else
r++;
printf("\n enter your value:");
scanf("%d",&q[r]);
}
}
void delet()
{
if(f==-1)
{
printf("\n under flow :");
}
else
{

printf("\n%d",q[f]);
if(r==f)
{
f=-1;
r=-1;
}
else
f++;
}

}
void display()
{
int i;
for(i=f;i<=r;i++)
{
printf("\t%d",q[i]);
}
}
void main()
{
int ch;
clrscr();
do
{
printf("\n1) for insert:");
printf("\n2) for delete:");
printf("\n3) for display:");
printf("\n4) for exit:");
printf("\n enter your chice:");
scanf("%d",&ch);
switch(ch)
{
case 1:insert();
break;
case 2:delet();
break;
case 3:display();
break;
case 4:break;
default:printf("\n invalid input:");
break;
}
}while(ch!=4);

}