import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int a = scan.nextInt();
        int b = scan.nextInt();
        for(int i = 0; i <= 1250; i++){
            if((int) (i * 0.08) == a && (int) (i * 0.1) == b){
                System.out.println(i);
                System.exit(0);
            }
        }
        System.out.println(-1);
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int a = scan.nextInt();
        int b = scan.nextInt();
        for(int i = 0; i <= 1250; i++){
            if((int) (i * 0.08) == a && (int) (i * 0.1) == b){
                System.out.println(i);
                System.exit(0);
            }
        }
        System.out.println(-1);
    }
} Main public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int a = scan.nextInt();
        int b = scan.nextInt();
        for(int i = 0; i <= 1250; i++){
            if((int) (i * 0.08) == a && (int) (i * 0.1) == b){
                System.out.println(i);
                System.exit(0);
            }
        }
        System.out.println(-1);
    } main {
        Scanner scan = new Scanner(System.in);
        int a = scan.nextInt();
        int b = scan.nextInt();
        for(int i = 0; i <= 1250; i++){
            if((int) (i * 0.08) == a && (int) (i * 0.1) == b){
                System.out.println(i);
                System.exit(0);
            }
        }
        System.out.println(-1);
    } Scanner scan = new Scanner(System.in); scan new Scanner(System.in) int a = scan.nextInt(); a scan.nextInt() scan.nextInt scan int b = scan.nextInt(); b scan.nextInt() scan.nextInt scan for(int i = 0; i <= 1250; i++){
            if((int) (i * 0.08) == a && (int) (i * 0.1) == b){
                System.out.println(i);
                System.exit(0);
            }
        } int i = 0; int i = 0; i 0 i <= 1250 i 1250 i++ i++ i {
            if((int) (i * 0.08) == a && (int) (i * 0.1) == b){
                System.out.println(i);
                System.exit(0);
            }
        } {
            if((int) (i * 0.08) == a && (int) (i * 0.1) == b){
                System.out.println(i);
                System.exit(0);
            }
        } if((int) (i * 0.08) == a && (int) (i * 0.1) == b){
                System.out.println(i);
                System.exit(0);
            } (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);
                System.exit(0);
            } System.out.println(i) System.out.println System.out System System.out i System.exit(0) System.exit System 0 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 scan = new Scanner(System.in);
        int a = scan.nextInt();
        int b = scan.nextInt();
        for(int i = 0; i <= 1250; i++){
            if((int) (i * 0.08) == a && (int) (i * 0.1) == b){
                System.out.println(i);
                System.exit(0);
            }
        }
        System.out.println(-1);
    }
} public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int a = scan.nextInt();
        int b = scan.nextInt();
        for(int i = 0; i <= 1250; i++){
            if((int) (i * 0.08) == a && (int) (i * 0.1) == b){
                System.out.println(i);
                System.exit(0);
            }
        }
        System.out.println(-1);
    }
} Main
import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int a = scan.nextInt();
        int b = scan.nextInt();
        for(int i = 0; i <= 1250; i++){
            if((int) (i * 0.08) == a && (int) (i * 0.1) == b){
                System.out.println(i);
                System.exit(0);
            }
        }
        System.out.println(-1);
    }
}