#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <math.h>
using namespace std;
typedef unsigned int uint;
typedef unsigned long ulong;

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

	cin >> xa >> ya >> xb >> yb;

	double answer = ya + (yb - ya) * xa / (xa + xb);

	printf("%.6lf",answer);

	return 0;
}