#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  cout << fixed << setprecision(15);
  // min sqrt(a^2 + (y - b)^2) + sqrt(c^2 + (y - d)^2)
  // (a, b) - (-c, d) を結ぶ直線と x = 0 の交点になれば OK
  // b = at + y, d = -ct + y
  // b - d = (a + c) t -> t = (b - d) / (a + c)
  // y = -at + b = (-ab + ad + ab + bc) / (a + c) = (ad + bc) / (a + c)
  // y = ct + d = (bc - cd + ad + cd) / (a + c)
  cout << (double) (a * d + b * c) / (a + c) << endl;
  return 0;
}