#include <iostream>
#include <vector>
#include <algorithm>
#include <climits>
#include <cmath>
#include <iomanip>
using namespace std;

#define vi vector<int>
#define vii vector<vector<int>>
#define ll long long int

void testcase(){
    double a,b,c,d;
    cin>>a>>b>>c>>d;
    cout << setprecision(12) << (a * d + c * b)/(b + d) << "\n";
}

int main(){
    ios_base :: sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    testcase();
    return 0;
}
#include<bits/stdc++.h>

using namespace std;

#define ll long long int
#define get(a) ll a ; cin >> a ;
#define show(a) cout << a << endl;

int main()
{
	ios_base::sync_with_stdio(false);
	cout << std::setprecision(12);
    cin.tie(NULL);

	get(sx);
	get(sy);

	get(gx);
	get(gy);

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

	cout << answer << endl;

	return 0;
}