import java.util.Scanner;

class no236{
	public static void main(String[] args) {
		Scanner stdIn=new Scanner(System.in);
		double a=stdIn.nextDouble(),b=stdIn.nextDouble(),x=stdIn.nextDouble(),y=stdIn.nextDouble();
		System.out.print(x/a*b<=y?x+x*b/a:y+y*a/b);
	}
}