#include<bits/stdc++.h>
using namespace std;
double k;
double a,b,c,d,s,p;
double f(double x)
{
	return -b/k+a;
}
int main()
{
	
    cin>>a>>b>>c>>d;
    b=-b;
    k=(b-d)/(a-c);
    s=f(0);
    printf("%.10lf",s);
	return 0;
	
}
#include<bits/stdc++.h>
#define loop(i, n) for(int i = 0; i < n; i++)
#define pb          push_back
#define ll          long long int
#define vi          vector<int>
#define pi          pair<int, int>
#define W(t)        int t; cin >> t; while(t--)
#define FIO         ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define ff          first
#define ss          second
#define rep(i, a, b)for(ll i = a; i <= b; i++)
#define dep(i, a, b)for(ll i = a; i >= b; i--) 
#define all(x)      x.begin(), x.end()
#define tr(it, a) for(auto it = a.begin(); it != a.end(); it++)
#define PI                3.14159265
using namespace std;
const ll mod = (ll)(998244353);
vector<ll> read(int n){
    
    vector<ll> v(n);

    loop(i, n)
        cin >> v[i];
    return v;
}
vector<ll> fact, invf;
ll powmod(ll a, ll b) {
    ll res = 1;
    a %= mod;
    for (; b; b >>= 1) {
        if (b & 1)
            res = res * a % mod;
        a = a * a % mod;
    }
    return res;
}
 
void build(int n) {
    fact = vector<ll>(n+1, 1);
    invf = vector<ll>(n+1, 1);
    for (int i = 2; i <= n; i++)
        fact[i] = fact[i-1] * i % mod;
    invf[n] = powmod(fact[n], mod-2);
    for (int i = n-1; i >= 2; i--)
        invf[i] = invf[i+1] * (i+1) % mod;
}
 
ll nChoosek(int n, int k) {
    if (n < k)
        return 0;
    return fact[n] * invf[k] % mod * invf[n-k] % mod;
}
ll test = 0;
void solve(){
    ll i, j, k, m, n, cnt = 0, ans = 0, sum = 0;

   double sx, sy, gx, gy;

   cin >> sx >> sy >> gx >> gy;

   double res = (gy*sx + gx*sy)/(sy+gy);

   cout <<fixed << setprecision(10) << res << "\n";


    test++;
}
    
int main(){
    FIO
    
    #ifndef ONLINE_JUDGE
	freopen("C:/Codeforces/input.txt", "r", stdin);
	freopen("C:/Codeforces/output.txt", "w", stdout);
	#endif
    ll i = 1;
    //W(t)
    solve();
    
    return 0;
}