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 = 0; i <= 1100; i++){
        	if((int)(i*0.08)==A && (int)(i*0.1)==B){
            	System.out.println(i);
              	return;
            }
        }
      	System.out.println(-1);
    }
} 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 = 0; i <= 1100; i++){
        	if((int)(i*0.08)==A && (int)(i*0.1)==B){
            	System.out.println(i);
              	return;
            }
        }
      	System.out.println(-1);
    }
} Main public static void main(String[] args){
    	Scanner sc = new Scanner(System.in);
      	int A = sc.nextInt();
      	int B = sc.nextInt();
      
      	for(int i = 0; i <= 1100; i++){
        	if((int)(i*0.08)==A && (int)(i*0.1)==B){
            	System.out.println(i);
              	return;
            }
        }
      	System.out.println(-1);
    } main {
    	Scanner sc = new Scanner(System.in);
      	int A = sc.nextInt();
      	int B = sc.nextInt();
      
      	for(int i = 0; i <= 1100; i++){
        	if((int)(i*0.08)==A && (int)(i*0.1)==B){
            	System.out.println(i);
              	return;
            }
        }
      	System.out.println(-1);
    } 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 = 0; i <= 1100; i++){
        	if((int)(i*0.08)==A && (int)(i*0.1)==B){
            	System.out.println(i);
              	return;
            }
        } int i = 0; int i = 0; i 0 i <= 1100 i 1100 i++ i++ i {
        	if((int)(i*0.08)==A && (int)(i*0.1)==B){
            	System.out.println(i);
              	return;
            }
        } {
        	if((int)(i*0.08)==A && (int)(i*0.1)==B){
            	System.out.println(i);
              	return;
            }
        } if((int)(i*0.08)==A && (int)(i*0.1)==B){
            	System.out.println(i);
              	return;
            } (int)(i*0.08)==A && (int)(i*0.1)==B (int)(i*0.08)==A (int)(i*0.08) i 0.08 A (int)(i*0.1)==B (int)(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 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 = 0; i <= 1100; i++){
        	if((int)(i*0.08)==A && (int)(i*0.1)==B){
            	System.out.println(i);
              	return;
            }
        }
      	System.out.println(-1);
    }
} 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 = 0; i <= 1100; i++){
        	if((int)(i*0.08)==A && (int)(i*0.1)==B){
            	System.out.println(i);
              	return;
            }
        }
      	System.out.println(-1);
    }
} 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 = 0; i <= 1100; i++){
        	if((int)(i*0.08)==A && (int)(i*0.1)==B){
            	System.out.println(i);
              	return;
            }
        }
      	System.out.println(-1);
    }
}