Factorial program in c using recursion

#include<stdio.h>
long factorial(int);
int main(){
   int n;
   long f;
   printf("Enter an integer to find factorial\n");
   scanf("%d", &n); 
   if (n < 0)
      printf("Negative integers are not allowed.\n");
   else{
      f = factorial(n);
      printf("%d! = %ld\n", n, f);
   }
   return 0;
}
long factorial(int n){
   if (n == 0)
     return 1;
   else
     return(n * factorial(n-1));
}