#include <cstdio>
#include <cmath>
#include <cstring>
#include <iostream>
#include <iomanip>
#include <sstream>
#include <map>
#include <set>
#include <array>
#include <queue>
#include <string>
#include <vector>
#include <numeric>
#include <algorithm>
#include <functional>
#define MOD 1000000007
#define INF 11234567890
#define in std::cin
#define out std::cout
#define rep(i,N) for(LL i=0;i<N;++i)
typedef long long int LL;
typedef std::pair<int, int> P;

double A, B, X, Y;
double res1, res2;

int main()
{
	in >> A >> B >> X >> Y;

	res1 = X / A*(A + B);
	res2 = Y / B*(A + B);
	res1 = (res1 > X + Y ? -1 : res1);
	res2 = (res2 > X + Y ? -1 : res2);

	out << std::fixed << std::setprecision(10);
	out << std::max(res1, res2) << std::endl;
	return 0;
}