#include<stdio.h>
#include<math.h>
int main(){
    double x,y,ka,c,m,n;
    while(scanf("%lf%lf%lf%lf",&ka,&c,&m,&n)!=EOF){
    if(ka==0&&c==0&&m==0&&n==0) break;
    x=sqrt(ka*ka+4*m*n*c*ka);
    y=(-ka+x)/(2*m*n);
    printf("%.3lf\n",-log10(y*m));
    }
return 0;
}