package m.shin;
import java.util.Scanner;
public class Con236 {

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		double a = s.nextDouble() , b = s.nextDouble() ,x = s.nextDouble() , y = s.nextDouble();
		s.close();

		if(x / a * b >= y){
			System.out.println(y + y * a / b);
		}else{
			System.out.println(x + x * b / a);
		}

	}

}