int x;
int y;

main(int argc, char *argv[]) {
  if (argc != 3) {
    fprintf(stderr, "usage: print_sum x y\n");
    exit(1);
  }
  if (sscanf(argv[1], "%d", &x) != 1) {
    fprintf(stderr, "usage: print_sum x y -- x must be an integer\n");
    exit(1);
  }
  if (sscanf(argv[2], "%d", &y) != 1) {
    fprintf(stderr, "usage: print_sum x y -- y must be an integer\n");
    exit(1);
  }
  printf("%d + %d = %d\n", x, y, sum());
}