#include <bits/stdc++.h>
using namespace std;
const int INF = 1000000000;
const int MOD = 1000000000 + 7;
const int N = 1e5 + 10;
int v1, v2, d, L;
int main() {
  scanf("%d%d%d%d", &d, &L, &v1, &v2);
  printf("%.10lf\n", (double)1.0 * (L - d) / (v1 + v2));
  return 0;
}
