#include<stdio.h>
int main(){
	double sum=0;
	for(int i=0;;i++)
	{
		double term =1.0/(i*2+1);
		if(i%2==0) sum+=trem;
		else 
		  sum-=term;
		if(term<1e-6) break;
	}
	printf("%.6f\n",sum);
	return 0;
	
}