import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
public class Main {

	 public static void main(String[] args) {
	      BufferedReader stdReader =new BufferedReader(new InputStreamReader(System.in));
	      try {
	    	  String[] line1 = stdReader.readLine().split(" ");
	    	  String[] line2 = stdReader.readLine().split(" ");
	    	  int x1 = Integer.parseInt(line1[0]);
	    	  int y1 = Integer.parseInt(line1[1]);
	    	  int x2 = -1*Integer.parseInt(line2[0]);
	    	  int y2 = Integer.parseInt(line2[1]);
	    	  
	    	  double slope = (double)(y2-y1) / (x2-x1);
	    	  double intercept = y2-slope*x2;	    	  
	    	  System.out.println(intercept);
	      } catch (IOException e) {
			e.printStackTrace();
	      }
	}
}