/* -*- coding: utf-8 -*-
 *
 * 306.cc: No.306 さいたま2008 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

/* typedef */

/* global variables */

/* subroutines */

/* main */

int main() {
  int xa, ya, xb, yb;
  cin >> xa >> ya >> xb >> yb;

  double y = (double)(yb - ya) / (xb + xa) * xa + ya;
  printf("%.8lf\n", y);
  return 0;
}