#include <assert.h>
#include <stdio.h>
#include <stdlib.h>

/* Helper function that computes the product.  Returns the
 * sum 1 + 2 + 3 + ... + n.  Assumes that n > 0.
 */
int _sum(int n) {
  // Base case: the sum 1 is just 1.
  if (n == 1) return 1;
  // Recursive case: compute (1 + 2 + ... + (n-1)) + n.
  return _sum(n-1) + n;
}

/* Interface for computing the sum
 *   1 + 2 + 3 + ... + n
 * Returns -1 if n <= 0 or s is NULL; otherwise, stores the
 * sum in the cell that s references and returns 0.
 */
int sum(int n, int * s) {
  if (n <= 0 || s == NULL) 
    return -1;

  // We know that n > 0 at this point, so we can safely 
  // call the helper function.
  *s = _sum(n);

  // success
  return 0;
}

int main() {
  // test the sum function
  int s, err;
  err = sum(5, &s);
  assert (err == 0);
  // print the result to the console
  printf("%d\n", s);
  // test bad input
  err = sum(-3, &s);
  assert (err != 0);
  return 0;
}