import java.util.Scanner;

class Pair implements Comparable<Pair> {
  int x, y;
  public Pair(String s) {
    String[] tmp = s.split("\\.");
    this.x = Integer.parseInt(tmp[0]);
    this.y = Integer.parseInt(tmp[1]);
  }
  
  @Override
  public int compareTo(Pair other) {
    if (this.x != other.x) return this.x - other.x;
    return this.y - other.y;
  }
}

public class whichoneislarger_mn {

  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String num1 = sc.nextLine();
    String num2 = sc.nextLine();
    Pair p1 = new Pair(num1);
    Pair p2 = new Pair(num2);
    double d1 = Double.parseDouble(num1), d2 = Double.parseDouble(num2);
    if (p1.compareTo(p2) < 0 && d1 < d2) {
      System.out.println(d2);
    } else if (p2.compareTo(p1) < 0 && d2 < d1) {
      System.out.println(d1);
    } else {
      System.out.println(-1);
    }
    sc.close();
  }

  
}