B.cpp

/*
 © 2021-01-21 01:17:56 Franco1010 All Rights Reserved
*/

// #pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math")
// #pragma GCC target("avx,avx2,fma")
#include <bits/stdc++.h>
#define f first
#define s second
#define fore(i,a,b) for(int i = (a), ThxMK = (b); i < ThxMK; ++i)
#define pb push_back
#define all(s) begin(s), end(s)
#define _ ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define sz(s) int(s.size())
#define ENDL '\n'
using namespace std;
typedef long double ld;
typedef long long lli;
typedef pair<lli,lli> ii;
typedef pair<double,double> dd;
typedef vector<lli> vi;
#define deb(x) cout << #x": " << (x) << endl;
int main(){ _
  // freopen("file.in","r",stdin);
  // freopen("file.out","w",stdout);
  dd s, g; cin >> s.f >> s.s >> g.f >> g.s;
  double b = (g.f - s.f) / ((g.s / s.s) + double(1));
  cout << fixed << setprecision(6);
  cout << s.f + b << ENDL;
  return 0;
}