#include<bits/stdc++.h>
#define pb       push_back
#define f        first
#define s       second
#define pi       acos(-1.0)
#define mod      1000000007
#define test     int t; cin>>t ;while(t--)
typedef long long ll;
using namespace std;

int main()
{

       
       
      ll a,b,c,d;
      cin>>a>>b>>c>>d;
      
       double ans;
      ans=(1.*b*(c-a))/(d+b);
      ans+=a;
      printf("%.10lf",ans);
        
     
     return 0;
      
}


#include <iostream>
#include <iomanip>
using namespace std;

int main() {
    int sx, sy, gx, gy;
    double res, slope;
    cin >> sx >> sy >> gx >> gy;
    // gのy座標を反転
    gy = -gy;
    // グラフの傾きの逆数 * -1
    slope = -(double)(sx - gx) / (double)(sy - gy);
    res = (double)sx + (double)sy * slope;
    cout << fixed << setprecision(10) << res << endl;
    return 0;
}