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


#define vt vector
#define all(c) (c).begin(), (c).end()
#define sz(x) (int)(x).size()
#define ll long long
#define ld long double
#define ar array
#define inf() ifstream cin("d.txt")
#define onf() ofstream cout("test.txt")
#define mod 1000000007
#define pb push_back
#define endl cout<<"\n";
#define prt1(a) cout<<a<<" "
#define prt2(a,b) cout<<a<<" "<<b<<" "
#define prt3(a,b,c) cout<<a<<" "<<b<<" "<<c<<" "
#define in(a) cin>>a
#define inn(a,b) cin>>a>>b
#define innn(a,b,c) cin>>a>>b>>c 
#define rep(i,st,end) for(int i=st;i<end;i++)
#define repv(i,st,end) for(int i=st;i>=end;i--)
#define fio ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define TEST ll tc;cin>>tc;while(tc--)
#define EACH(x, a) for (auto& x: a)

template<class A> void read(vt<A>& v);
template<class A, size_t S> void read(ar<A, S>& a);
template<class T> void read(T& x) {
    cin >> x;
}
void read(double& d) {
    string t;
    read(t);
    d=stod(t);
}
void read(long double& d) {
    string t;
    read(t);
    d=stold(t);
}
template<class H, class... T> void read(H& h, T&... t) {
    read(h);
    read(t...);
}
template<class A> void read(vt<A>& x) {
    EACH(a, x)
        read(a);
}
template<class A, size_t S> void read(array<A, S>& x) {
    EACH(a, x)
        read(a);
}
#define debug(args...) { string _s = #args; replace(_s.begin(), _s.end(), ',', ' '); stringstream _ss(_s); istream_iterator<string> _it(_ss); err(_it, args); }

void err(istream_iterator<string> it) {}
template<typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
    cerr << *it << " = " << a <<"\n";
    err(++it, args...);
}


void solve()
{
    ll a,b,c,d;
    read(a,b,c,d);
    double x = ((a*d) + (b*c))/((b+d)*1.0);
    cout<<fixed <<setprecision(9)<<x;

}

 int main()
 {
    clock_t start, end; 
    start = clock(); 
    fio;
    ll k = 1;
    ll tc = 1;
    //cin>>tc;
    while(tc--){
       // cout<<"Case #"<<k<<": ";k++;
        solve();
        endl;
    }
     double time_taken = double(end - start) / double(CLOCKS_PER_SEC); 
    // debug(time_taken);
    
 }



#include <algorithm>
#include <bitset>
#include <cmath>
#include <cctype>
#include <fstream>
#include <functional>
#include <iostream>
#include <iomanip>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <stdio.h>
#include <string>
#include <sstream>
#include <stack>
#include <set>
#include <vector>

#define rep(i,a,b) for((i)=a;i<(int)(b);i++)

const int AAA = 1000000007;


using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pairII;
typedef vector<int> vecI;

ll gcd(ll a, ll b) {
	if (b == 0)return a;
	return gcd(b, a % b);
}

ll lcm(ll a, ll b) {
	return a / gcd(a, b) * b;
}
ll powD(ll a, ll b) {
	ll ans = 1;
	a %= AAA;
	for (int i = 0; i < b; i++)
	{
		ans *= a;
		ans %= AAA;
	}
	return ans;
}

vecI input(int N) {
	int i;
	vecI A(N);
	rep(i, 0, N) {
		cin >> A[i];
	}
	return A;
}

int main(int argc, char* argv[]) {
	int i, j;
	int N, M, K;

	double Sx, Sy, Gx, Gy;
	cin >> Sx >> Sy >> Gx >> Gy;
	double k = (-Gy - Sy) / (Gx - Sx);
	double ans = Sx - Sy / k;
	printf("%.12f", ans);

	return 0;
}
