#include <iostream>
#include <string>
#include <fstream>
#include <iomanip>
int main()
{
	double a, b, coffee, tea;
	std::cin >> a;
	std::cin >> b;
	std::cin >> coffee;
	std::cin >> tea;

	double ret;

	if(a > b)
	{
		if(coffee * b / a > tea)
		{
			ret = tea + tea * a / b;
		}
		else
		{
			ret = coffee + coffee * b / a;
		}
	}
	else
	{
		if(tea * a / b > coffee)
		{
			ret = coffee + coffee * b / a;
		}
		else
		{
			ret = tea + tea * a / b;
		}
	}

	std::cout << std::setprecision(12) << ret << std::endl;	
}