#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, a, b, t;
  cin >> n >> t >> a >> b;
  a += b;
  t -= n;
  cout << setprecision(7) << fixed;
  long double l = t * 1.0 / a;
  cout << l;
  return 0;
}
