import java.util.Locale;

public class Euler235 {
    static double seriesSum(int n, double r) {
        double sum = 0.0;
        double power = 1.0;
        for (int k = 1; k <= n; ++k) {
            sum += (900.0 - 3.0 * k) * power;
            power *= r;
        }
        return sum;
    }

    public static String solve() {
        int n = 5000;
        double target = -600000000000.0;

        double lo = 1.0;
        double hi = 1.2;

        for (int it = 0; it < 200; ++it) {
            double mid = (lo + hi) / 2.0;
            double s = seriesSum(n, mid);
            if (s > target) {
                lo = mid;
            } else {
                hi = mid;
            }
        }

        double ans = (lo + hi) / 2.0;
        return String.format(Locale.US, "%.12f", ans);
    }

    public static void main(String[] args) {
        System.out.println(solve());
    }
}