package yukicoder236;
import java.util.Scanner;
public class Main {
	public static void main(String[] args){
		Scanner sc=new Scanner(System.in);
		double a=sc.nextDouble();
		double b=sc.nextDouble();
		double x=sc.nextDouble();
		double y=sc.nextDouble();
		if(x*b>=y*a){
			System.out.println(y+(y/b)*a);
		}else if(x*b<y*a){
			System.out.println(x+(x/a)*b);
		}
	}
}