#include #include #include #include #include #include #include #include #include #include using namespace std; typedef long long ll; typedef unsigned long long ull; typedef pair pii; typedef vector vi; int main() { long double x; cin >> x; long double ans = 0; for (int i = 1000000000; i >= 1; i--) { ans += 1/(x+i)/(x+i); } printf("%.12Lf\n", ans); return 0; }