package yukicoder; import java.util.*; public class Q454 { public static void main(String[] args) { Scanner sc = new Scanner(System.in); double res = Math.PI * Math.PI / 6; double ans = 0; double i = 1; double x = sc.nextDouble(); ans = 0; i = 1; int d = (int) x; for (double j = 1; j <= d; ++j) res -= 1 / (j * j); for (; res - res * Math.pow(Math.E, -x / i) > 0.0000000001 || i <= 30; ++i) { ans += 1 / ((i + x) * (i + x)); res -= 1 / ((i + d) * (i + d)); } ans += res; System.out.printf("%.20f\n", ans); } }