// yukicoder: No.454 逆2乗和
// 2019.7.21 bal4u

#include <stdio.h>

#define LIM 100000

int main()
{
	int n;
	double x, t, ans;
	
	scanf("%lf", &x);
	ans = 1.0/(x+LIM), t = (x+LIM)*(x+LIM);
	for (n = LIM; n > 1; n--) {
		t += 1-2*(x+n);
		ans += 1.0/t;
	}
	printf("%.15lf\n", ans);
	return 0;
}