#include
#include
double f(double a,double b,double c){
double w=sqrt(b*b-4*a*c);
return (-b+w)/(2.0*a);
}
int main(){
double ka,ori,m,n;
while(scanf("%lf%lf%lf%lf",&ka,&ori,&m,&n)&&
(ka||ori||m||n)){
(ka||ori||m||n)){
double x=f(m*n,ka,-1*ori*ka);
printf("%.3f\n",-log10(x*m));
}
return 0;
}