> #include <stdio.h>
> #include <math.h>
> 
> double ka;
> double acid;
> double res;
> double temp, delta;
> int m, n;
> 
> int main()
> {
> 	scanf("%lf%lf%d%d", &ka, &acid, &m, &n);
> 	while(ka && acid && m && n)
> 	{
> 		delta = sqrt(ka * ka+4 * n * acid * ka * m);
>         temp = (-ka + delta) / (2 * n);
>         res = -log10(temp);
>         printf("%.3lf\n",res);
> 		scanf("%lf%lf%d%d", &ka, &acid, &m, &n);
> 	}
> 	return 0;
> }