int sgn (double x0) {
	if (x0 > 0)
		return 1;
	else
		if (x0 == 0)
			return 0;
		else
			return -1;
}
double slagarctg (double x0, int k0) {
	if (fabs (x0) < 1)
		return (pow (-1, k0)*pow (x0, 2 * k0 + 1)) / (2 * k0 + 1);
	else
		if (fabs (x0)>=1)
		return( pow (-1, k0)*pow (x0, -(2 * k0 + 1))) / (2 * k0 + 1);
}
double arctgM (double x0) {
	int k = 0;
	double arctag = 0;
	double slag = 0;
	do{
		slag = slagarctg (x0, k++);
		arctag += slag;
		std::cout << " C = " << arctag << "         slag = " << slag << std::endl;
	} while (fabs (slag)>E1);
	double arctag1 = arctag;
	if (fabs (x0) >= 1)
		arctag1 = PI / 2 * sgn (x0) - arctag;
	return arctag1;
}