//嘘解法
#include <stdio.h>

int main() {
	double x;
	
	scanf("%lf", &x);
	
	double ans = 0;
	
	for (int i = 1; i < 100000000; i++) {
		ans += 1 / ((x + i) * (x + i));
	}
	ans += 1e-8;
	printf("%.14lf\n", ans);
	return 0;
}