#include<iostream> #include<iomanip> #include<vector> #include<string> using namespace std; int main(){ int x1,x2,y1,y2; double y; cin>>x1>>y1>>x2>>y2; y=(double)(x1*y2+x2*y1)/(x1+x2); cout<<fixed<<setprecision(6)<<y<<endl; return 0; }