Program to count the number of lower case vowels in a string

#include<iostream.h>
#include<conio.h>
#include<stdlib.h>
#include<string.h>
#include<math.h>
#include<ctype.h>
void main()
{
 clrscr();
 char st[100];
 int l,n,i;
 cout<<"Enter the string\n";
 cin.getline(st,100);
 l=strlen(st);
 for(i=0;i<=l;i++)
 {
   if(islower(st(i))) && ((st(i)==a)||(st(i)==e)||(st(i)==i)||(st(i)==o)||(st(i)==u))
   n++;
 }
 cout<<"The number of lower case vowels is : "<<n;
 getch();
}