#include <stdio.h>
#include <math.h>

int main() {
	double x;
	scanf("%lf", &x);

	int n;
	double ans = 0.0;

	for (n = 1; n < 100000000; n++) {
		ans += 1.0 / ((x + n) * (x + n));
	}

	printf("%.10lf\n", ans);
	return 0;
}