import java.util.*;
public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    for(int i=1; i<10000;i++){
      if((int)Math.floor(i*0.08)==A&&(int)Math.floor(i*0.1)==B){
        System.out.println(i);
        return;
      }
    }
    System.out.println(-1);
    return;
  }
} import java.util.*; util.* java public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    for(int i=1; i<10000;i++){
      if((int)Math.floor(i*0.08)==A&&(int)Math.floor(i*0.1)==B){
        System.out.println(i);
        return;
      }
    }
    System.out.println(-1);
    return;
  }
} Main public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    for(int i=1; i<10000;i++){
      if((int)Math.floor(i*0.08)==A&&(int)Math.floor(i*0.1)==B){
        System.out.println(i);
        return;
      }
    }
    System.out.println(-1);
    return;
  } main {
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    for(int i=1; i<10000;i++){
      if((int)Math.floor(i*0.08)==A&&(int)Math.floor(i*0.1)==B){
        System.out.println(i);
        return;
      }
    }
    System.out.println(-1);
    return;
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int A = sc.nextInt(); A sc.nextInt() sc.nextInt sc int B = sc.nextInt(); B sc.nextInt() sc.nextInt sc for(int i=1; i<10000;i++){
      if((int)Math.floor(i*0.08)==A&&(int)Math.floor(i*0.1)==B){
        System.out.println(i);
        return;
      }
    } int i=1; int i=1; i 1 i<10000 i 10000 i++ i++ i {
      if((int)Math.floor(i*0.08)==A&&(int)Math.floor(i*0.1)==B){
        System.out.println(i);
        return;
      }
    } {
      if((int)Math.floor(i*0.08)==A&&(int)Math.floor(i*0.1)==B){
        System.out.println(i);
        return;
      }
    } if((int)Math.floor(i*0.08)==A&&(int)Math.floor(i*0.1)==B){
        System.out.println(i);
        return;
      } (int)Math.floor(i*0.08)==A&&(int)Math.floor(i*0.1)==B (int)Math.floor(i*0.08)==A (int)Math.floor(i*0.08) Math.floor Math i*0.08 i 0.08 A (int)Math.floor(i*0.1)==B (int)Math.floor(i*0.1) Math.floor Math i*0.1 i 0.1 B {
        System.out.println(i);
        return;
      } System.out.println(i) System.out.println System.out System System.out i return; System.out.println(-1) System.out.println System.out System System.out -1 1 return; String[] args args public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    for(int i=1; i<10000;i++){
      if((int)Math.floor(i*0.08)==A&&(int)Math.floor(i*0.1)==B){
        System.out.println(i);
        return;
      }
    }
    System.out.println(-1);
    return;
  }
} public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    for(int i=1; i<10000;i++){
      if((int)Math.floor(i*0.08)==A&&(int)Math.floor(i*0.1)==B){
        System.out.println(i);
        return;
      }
    }
    System.out.println(-1);
    return;
  }
} Main
import java.util.*;
public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    for(int i=1; i<10000;i++){
      if((int)Math.floor(i*0.08)==A&&(int)Math.floor(i*0.1)==B){
        System.out.println(i);
        return;
      }
    }
    System.out.println(-1);
    return;
  }
}