#include<stdio.h>
double power(double x, int n)
{
	double res = 1;

	for (int i = 0; i < n; i++)
	{
		res *= x;
	}

	return res;
}

void abs(double* b)
{
	if((*b)<0)
		(*b)*= -1;
}

double term(double x, int n)
{
	int a = 0, b = 0;
	double c = 0;

	a = power(-1, n);
	b = 2 * n + 1;
	c = power(x, b);

	return((a*c / b));
}

double arctan(double x)
{
	abs(&x);
	double res = 0;
	int n = 0;
	double t = 9999;

	while (t >= 0.0001)
	{
		t = term(x, n);

		res += t;
		n++;
		abs(&t);
	}

	return res;
}



int main()
{
	double res;
	double x;

	//scanf_s("%lf", &x);

	res = arctan(0.5);
	printf("%.5lf\n", res);

	return 0;
}