参考答案：

#include<iostream>

using namespace std;

const double INF = 1e9;

int main()
{
    double n, l = -INF, r = INF;

    cin >> n;
    while (r - l > 1e-8)
    {
        double mid = (l + r) / (2.0);
        if (mid * mid * mid >= n) r = mid;
        else l = mid;
    }

    printf("%.6lf", l);

    return 0;
}