#include <bits/stdc++.h>      
using namespace std;            

#define    int               long long int
#define  ld         long double
 #define  f(i,j,n)            for(int i = j; i <= n; i++)
#define  r(i,n,j)       for(int i = n; i >= j; i--)
 #define  all(container)       container.begin() , container.end()
#define  sz(container)   (int)container.size()
 #define  ff             first
#define  ss             second
 #define  pii      pair <int , int>
#define  sp(x)       setprecision(x)
#define  mod  1000000007
#define  endl  "\n"
 #define  pb    push_back
#define  mp    make_pair
 #define  T     int ttt; cin >> ttt; while(ttt--)
#define fast ios_base::sync_with_stdio(false),cin.tie(NULL),cout.tie(NULL);

int power(int x, int y, int p)  
{  
    int res = 1;    
    x = x % p; 
    if (x == 0) return 0; 
    while (y > 0)  
    {     
        if (y & 1)  
            res = (res*x) % p;  
        y = y>>1; 
        x = (x*x) % p;  
    }  
    return res; 
}

int32_t main()
{
    fast
    ld x,y,a,b;
    cin>>x>>y>>a>>b;
    if(x>a) {
        swap(x,a);
        swap(y,b);
    }
    ld d=abs(x-a);
    ld m= b*d;
    m=m/(ld)(y+b);
    cout<<fixed<<sp(10)<<a-m;
}
#include<bits/stdc++.h>
using namespace std;
typedef int64_t ll;
typedef long double ld;
const ll MOD=1000000007;
const ll MODA=998244353;
#define rep(i,n) for(ll i=0;i<(ll)(n);i++)
long long gcd(long long a,long long b){
    ll gcdmax=max(a,b);
    ll gcdmin=min(a,b);
    while(true){
        if(gcdmax%gcdmin==0)break;
        else gcdmax%=gcdmin;
        swap(gcdmin,gcdmax);
    }
    return gcdmin;
}
ll powerup(ll N,ll P,ll M){
    if(P==0)return 1;
    else if(P%2==0){
        ll t=powerup(N,P/2,M);
        return t*t%M;
    }
    else return N*powerup(N,P-1,M)%M;
}
vector<ll> find_divisor(ll N){
  ll k=1;
  while(k*k<=N){
    k++;
  }
  vector<ll> A(1);
  rep(i,k){
    if(i==1)A.at(0)=1;
    else if(i>=2){
      if(N%i==0)A.push_back(i);
    }
  }
  ll t=0;
  t=A.size();
  rep(i,t){
    if(A.at(t-i-1)*A.at(t-i-1)!=N)A.push_back(N/A.at(t-1-i));
  }
  return A;
}
vector<ll> fac;
vector<ll> finv;
vector<ll> inv;
void COMinit(ll N,ll P){
    rep(i,N+1){
        if(i==0){
            fac.push_back(1);
            finv.push_back(1);
            inv.push_back(1);
        }
        else if(i==1){
            fac.push_back(1);
            finv.push_back(1);
            inv.push_back(1);
        }
        else{
            fac.push_back(fac.at(i-1)*i%P);
            inv.push_back(P-inv.at(P%i)*(P/i)%P);
            finv.push_back(finv.at(i-1)*inv.at(i)%P);
        }
    }
}

ll COM(ll n,ll k,ll P){
    if(n<k)return 0;
    if(n<0||k<0)return 0;
    return fac.at(n)*(finv.at(k)*finv.at(n-k)%P)%P;
}


struct UnionFind {
    vector<ll> par; // par[i]:iの親の番号　(例) par[3] = 2 : 3の親が2

    UnionFind(ll N) : par(N) { //最初は全てが根であるとして初期化
        for(ll i = 0; i < N; i++) par[i] = i;
    }

    ll root(ll x) { // データxが属する木の根を再帰で得る：root(x) = {xの木の根}
        if (par[x] == x) return x;
        return par[x] = root(par[x]);
    }

    void unite(ll x, ll y) { // xとyの木を併合
        ll rx = root(x); //xの根をrx
        ll ry = root(y); //yの根をry
        if (rx == ry) return; //xとyの根が同じ(=同じ木にある)時はそのまま
        par[rx] = ry; //xとyの根が同じでない(=同じ木にない)時：xの根rxをyの根ryにつける
    }

    bool same(ll x, ll y) { // 2つのデータx, yが属する木が同じならtrueを返す
        ll rx = root(x);
        ll ry = root(y);
        return rx == ry;
    }
};

int main(){
    ld sx,sy,gx,gy;
    cin>>sx>>sy>>gx>>gy;
    cout<<fixed<<setprecision(7)<<sx*(gy/(sy+gy))+gx*(sy/(sy+gy))<<endl;
}
