Factorial program in c using recursion

#include<stdio.h>
 
long factorial(int);
 
main()
{
   int num;
   long f;
 
   printf("ENTER A NUMBER TO FIND FACTORIAL :");
   scanf("%d",&num); 
 
   if(num<0)
      printf("NEGATIVE NUMBERS ARE NOT ALLOWED");
   else
   {
      f = factorial(num);
      printf("%d!=%ld",num,f);
   }
   return(0);
}
 
long factorial(int n)
{
   if(n==0)
      return(1);
   else
      return(n*factorial(n-1));
}