//#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
#include <iostream>
#include <algorithm>
#include <atcoder/modint>
using namespace std;
using namespace atcoder;
using mint = modint1000000007;
using ll = long long;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
using vi = vector<int>;  using vvi = vector<vi>;
using vl = vector<ll>;   using vvl = vector<vl>;
using vb = vector<bool>; using vvb = vector<vb>;
using vm = vector<mint>; using vvm = vector<vm>;
using vpi = vector<pii>; using vvpi = vector<vpi>;
using vpl = vector<pll>; using vvpl = vector<vpl>;
const int inf = 1 << 30;
const ll INF = 1LL << 60;
#define rep(i,m,n) for (int i = m; i < (int)(n); i++)
#define rrep(i,m,n) for (int i = m; i > (int)(n); i--)

int main(){
    cout << fixed << setprecision(10);
    int xa,ya,xb,yb; cin >> xa >> ya >> xb >> yb;
    cout << (xa*yb+xb*ya)/(long double)(xa+xb) << endl;
}