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

int main(){
   int xa,xb,ya,yb;
   cin >> xa >> ya >> xb >> yb;
   cout << fixed << setprecision(8) << abs(ya - yb) * min(xa,xb) / (xa + xb) + min(ya,yb) << endl;
}