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 < 10e4; i++) {
            int A2 = i * 8/100;
            int B2 = i * 10/100;
            if (A2==A && B2==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 sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();

        for (int i = 0; i < 10e4; i++) {
            int A2 = i * 8/100;
            int B2 = i * 10/100;
            if (A2==A && B2==B) {
                System.out.println(i);
                System.exit(0);
            }
        }

        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 < 10e4; i++) {
            int A2 = i * 8/100;
            int B2 = i * 10/100;
            if (A2==A && B2==B) {
                System.out.println(i);
                System.exit(0);
            }
        }

        System.out.println("-1");
    } main {
        Scanner sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();

        for (int i = 0; i < 10e4; i++) {
            int A2 = i * 8/100;
            int B2 = i * 10/100;
            if (A2==A && B2==B) {
                System.out.println(i);
                System.exit(0);
            }
        }

        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 < 10e4; i++) {
            int A2 = i * 8/100;
            int B2 = i * 10/100;
            if (A2==A && B2==B) {
                System.out.println(i);
                System.exit(0);
            }
        } int i = 0; int i = 0; i 0 i < 10e4 i 10e4 i++ i++ i {
            int A2 = i * 8/100;
            int B2 = i * 10/100;
            if (A2==A && B2==B) {
                System.out.println(i);
                System.exit(0);
            }
        } {
            int A2 = i * 8/100;
            int B2 = i * 10/100;
            if (A2==A && B2==B) {
                System.out.println(i);
                System.exit(0);
            }
        } int A2 = i * 8/100; A2 i * 8/100 i * 8 i 8 100 int B2 = i * 10/100; B2 i * 10/100 i * 10 i 10 100 if (A2==A && B2==B) {
                System.out.println(i);
                System.exit(0);
            } A2==A && B2==B A2==A A2 A B2==B B2 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" 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 < 10e4; i++) {
            int A2 = i * 8/100;
            int B2 = i * 10/100;
            if (A2==A && B2==B) {
                System.out.println(i);
                System.exit(0);
            }
        }

        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 < 10e4; i++) {
            int A2 = i * 8/100;
            int B2 = i * 10/100;
            if (A2==A && B2==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 sc = new Scanner(System.in);
        int A = sc.nextInt();
        int B = sc.nextInt();

        for (int i = 0; i < 10e4; i++) {
            int A2 = i * 8/100;
            int B2 = i * 10/100;
            if (A2==A && B2==B) {
                System.out.println(i);
                System.exit(0);
            }
        }

        System.out.println("-1");
    }
}
