注意[0,1], [1,+∞]

#include<iostream>
using namespace std;

int main() {
    double x;
    cin >> x;
    if (x < 0) {
        cout << "-";
        x = -x;
    }
    double l, r;
    if (x < 1) {
        l = x;
        r = 1;
    }
    else {
        l = 0;
        r = x;
    }
    while (r - l > 1e-8) {
        double mid = l + (r - l) / 2;
        if (mid * mid * mid >= x) r = mid;
        else l = mid;
    }
    printf("%.6lf", l);
    return 0;
}