#include <bits/stdc++.h>

int main() {
  double vl, vr;
  double d;
  double w;
  scanf("%lf %lf", &vl, &vr);
  scanf("%lf", &d);
  scanf("%lf", &w);

  printf("%.20lf\n", d * w / (vl + vr));

  return 0;
}