#include <algorithm>
#include <cstdio>
#include <iostream>
#include <map>
#include <cmath>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <stdlib.h>
#include <stdio.h>
#include <bitset>
using namespace std;
#define FOR(I,A,B) for(int I = (A); I < (B); ++I)
typedef long long ll;

int main(){
  double xa, ya, xb, yb;
  cin >> xa >> ya >> xb >> yb;
  double ans = yb - ((yb-ya)/(xb+xa))*xb;
  printf("%.08lf\n", ans);
  return 0;
}