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

int main()
{
	double xa, ya, xb, yb;
	double yp;
	cin >> xa >> ya >> xb >> yb;

	yp = ya + (yb - ya) * xa / (xb + xa);
	cout << fixed << setprecision(6) << yp << endl;

    return 0;
}