#include <bits/stdc++.h>

using namespace std;

struct Initializer {
  Initializer() {
    cin.tie(0);
    ios::sync_with_stdio(0);
    cout << fixed << setprecision(15);
  }
} initializer;

int main() {
  double xa, ya, xb, yb;
  cin >> xa >> ya >> xb >> yb;
  cout << (xa * yb + xb * ya) / (xa + xb) << endl;
}