int atou(char *s)
{
  int n = 0;
  for (char *p = s; *p; p++) {
    n *= 10;
    n += *p - '0';
  }
  return n;
}

int fact(int n)
{
  return n ? n * fact(n-1) : 1;
}

int factext(char *s)
{
  int n = atou(s);
  return fact(n);
}