#include<stdio.h>	
int main(void){	

	int 	i, n;
	i = 0;
	double  Sum = 0.0;		

	printf("Input positive integer:");
	scanf("%d",&n);

	for(i=0;i<=n;i++){

		if(i%2==0)
			Sum = Sum + (1.0/(2.0*i+1.0));
		else
			Sum = Sum - (1.0/(2.0*i+1.0));
	}	

	Sum = 4.0*Sum;

	printf("Sum is %.6f\n",Sum);

	return 0;

}