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

int main()
{
	double xa, ya, xb, yb;
	cin >> xa >> ya >> xb >> yb;
	cout << fixed << setprecision(10) << std::abs(xa*(yb - ya) / (xa + xb)) + ya << endl;
	return 0;
}