#include<stdio.h>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <algorithm>
#include <sstream>
#include <iostream>
#include <string>
#include <stdio.h>

using namespace std;

int main() {

	double A, B, X, Y;
	cin >> A >> B >> X >> Y;

	double m = -1.0;
	if (B*X <= A*Y) {
		m = max(m, X + X*(B/A));
	}
	if (B*X >= A*Y) {
		m = max(m, Y + Y*(A / B));
	}

	//printf_s("%.9lf\n", m);
	printf("%.9lf\n", m);

	return 0;
}