#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main()
{
    float solut(float a,float b,float c,float d);
    float a,b,c,d;
    scanf("%f,%f,%f,%f",&a,&b,&c,&d);
    printf("%10.7f\n",solut(a,b,c,d));
    return 0;
}
float solut(float a,float b,float c,float d)
{
    float x=1,x0,f,f1;
    do

    {
        x0=x;
        f=((a*x0+b)*x0+c)*x0+d;
        f1=(3*a*x0+2*b)*x0+c;
        x=x0-f/f1;
    }while(fabs(x-x0)>=1e-3);
    return (x);
}