#include <iostream>
#include <string>
#include <vector>
#include <cmath>
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <cstdio>
#include <functional>
#include <set>
#include <sstream>
#include <map>
#include <queue>
#include <stack>

using namespace std;

int main(){


    double a,b,c,d;
    cin>>a>>b>>c>>d;
    
    cout<<(a*d+b*c)/(a+c)<<endl;

    return 0;

}