#include <iostream>
#include <iomanip>
using namespace std;
int main() {
	double long A, B, X, Y;
	cin >> A >> B >> X >> Y;
	if (X * B / A <= Y) {
		cout << setprecision(15) << X + X* B / A << endl;
	}
	else {
		cout << setprecision(15) << Y + Y*A / B << endl;
	}
	return 0;
}