#include<math.h>
#include<stdio.h>
main()
{
	int a=1,b=1,c=1;
	float i=0.0,d=0.0001;
	while(fabs(1./a)>=fabs(d))
	{
		i=i+1/a;
		if(b%2!=0)
		c=-1;
		else c=1;
		a+=2;
		a=a*c;
		b++;
	 } 
	 printf("%f\n",i);
}