#include <math.h>

double rt3(double a)
{
	double x, new_x;

	x = a;

	while (1) {
		new_x = x - (x*x*x - a) / (3*x*x);
		if (fabs(new_x - x) < 1e-10) break;
		x = new_x;
	}

	return new_x;
}