#include <math.h>
#include <stdio.h>

int main(void) {

  // initialize variables
  double a, b, c, d;
  scanf("%lf %lf %lf %lf", &a, &b, &c, &d);

  // trinary expression magic. same as above
  (b * c - d < 1e-6 && b * c - d > -1e-6) ? printf("error")
                                          : printf("%.1lf", a / (b * c - d));

  return 0;
}