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();
    int ans = -1;
    for(int i=0; i<10000; i++){
      if((int)(i*0.08)==A) {
        if((int)(i*0.1)==B){
        ans = i;
        break;
        }
      }
    }
    System.out.println(ans);
  }
}
     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();
    int ans = -1;
    for(int i=0; i<10000; i++){
      if((int)(i*0.08)==A) {
        if((int)(i*0.1)==B){
        ans = i;
        break;
        }
      }
    }
    System.out.println(ans);
  }
} Main public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    int ans = -1;
    for(int i=0; i<10000; i++){
      if((int)(i*0.08)==A) {
        if((int)(i*0.1)==B){
        ans = i;
        break;
        }
      }
    }
    System.out.println(ans);
  } main {
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    int ans = -1;
    for(int i=0; i<10000; i++){
      if((int)(i*0.08)==A) {
        if((int)(i*0.1)==B){
        ans = i;
        break;
        }
      }
    }
    System.out.println(ans);
  } 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 int ans = -1; ans -1 1 for(int i=0; i<10000; i++){
      if((int)(i*0.08)==A) {
        if((int)(i*0.1)==B){
        ans = i;
        break;
        }
      }
    } int i=0; int i=0; i 0 i<10000 i 10000 i++ i++ i {
      if((int)(i*0.08)==A) {
        if((int)(i*0.1)==B){
        ans = i;
        break;
        }
      }
    } {
      if((int)(i*0.08)==A) {
        if((int)(i*0.1)==B){
        ans = i;
        break;
        }
      }
    } if((int)(i*0.08)==A) {
        if((int)(i*0.1)==B){
        ans = i;
        break;
        }
      } (int)(i*0.08)==A (int)(i*0.08) i 0.08 A {
        if((int)(i*0.1)==B){
        ans = i;
        break;
        }
      } if((int)(i*0.1)==B){
        ans = i;
        break;
        } (int)(i*0.1)==B (int)(i*0.1) i 0.1 B {
        ans = i;
        break;
        } ans = i ans i break; System.out.println(ans) System.out.println System.out System System.out ans 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();
    int ans = -1;
    for(int i=0; i<10000; i++){
      if((int)(i*0.08)==A) {
        if((int)(i*0.1)==B){
        ans = i;
        break;
        }
      }
    }
    System.out.println(ans);
  }
} public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int A = sc.nextInt();
    int B = sc.nextInt();
    int ans = -1;
    for(int i=0; i<10000; i++){
      if((int)(i*0.08)==A) {
        if((int)(i*0.1)==B){
        ans = i;
        break;
        }
      }
    }
    System.out.println(ans);
  }
} 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();
    int ans = -1;
    for(int i=0; i<10000; i++){
      if((int)(i*0.08)==A) {
        if((int)(i*0.1)==B){
        ans = i;
        break;
        }
      }
    }
    System.out.println(ans);
  }
}
    