//#include<fstream>
#include<iostream>
#include<vector>
using namespace std;
int main()
{
	//ifstream f("be.in");
	//int n;
	//f>>n;
	//for(int i=0;i<n;++i)
	//	f>>n;
	///*fstream f("log.txt", ios_base::out);
	//f<<M<<"\n";*/
	//f.close();
	int t1, t2, k1, k2;
	cin>>t1>>t2>>k1>>k2;
	double x=t1*k2+t2*k1;
	x/=(k1*k2);
	cout<<x<<"\n";
	return 0;
}