#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

int main()
{
  long double sx, sy, gx, gy;
  cin >> sx >> sy >> gx >> gy;

  long double ans = (sx * gy + sy * gx) / (sy + gy);
  cout << fixed << setprecision(10) <<ans;
}

#include <iostream>
#include <stdexcept>
#include <cassert>
#include <cmath>
#include <vector>
#include <string>
#include <iomanip>
using namespace std;


int main() {
  int sx, sy, gx, gy;
  cin >> sx >> sy >> gx >> gy;
  gy *= -1;

  cout << setprecision(12) << - (double) (gx - sx) / (gy - sy) * sy + sx << endl;
  return 0;
}
