#include <bits/stdc++.h>
using namespace std;
int main() {
  int d, l, v1, v2;
  cin >> d >> l >> v1 >> v2;
  double x = (double)(l - d);
  x = x / (v1 + v2);
  cout << fixed << setprecision(8) << x << "\n";
  return 0;
}
