#include <bits/stdc++.h>
using namespace std;
int main() {
  int d, L1 = 0, L2, v1, v2;
  scanf("%d %d %d %d", &d, &L2, &v1, &v2);
  double relv = v1 + v2;
  double move = L2 - d;
  double time = move / relv;
  printf("%lf\n", time);
  return 0;
}
