#include <bits/stdc++.h>
using namespace std;
int main() {
  int d, l, v1, v2;
  cin >> d >> l >> v1 >> v2;
  int x = l - d;
  double time = (double)x / (double)(v1 + v2);
  printf("%.20lf\n", time);
  return 0;
}
