import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    double xa = (-1) * sc.nextDouble();
    double ya = sc.nextDouble();
    double xb = sc.nextDouble();
    double yb = sc.nextDouble();
    double ans = (-1) * xa * (yb - ya) * (1 / (xb - xa)) + ya;
    System.out.println(ans);    
  }
}