#include <bits/stdc++.h>
using namespace std;
using namespace std;
const int N = 2005;
int _;
unsigned long long n;
set<int> qq;
int a[30];
int main() {
  int a, b, c, d;
  scanf("%d", &a), scanf("%d", &b), scanf("%d", &c), scanf("%d", &d);
  printf("%.10lf\n", (b - a) * 1.0 / (c + d));
  return 0;
}
