// yukicoder: No.454 逆2δΉ—ε’Œ // 2019.7.21 bal4u #include #define LIM 200000 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; }