import java.util.Scanner;

public class Main {
  public static void main(final String[] args) {
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    sc.close();

    int o = -1;
    for (int i = 1;i< (int)(100/0.08)+1;i++) {
      if ((int)(i*0.08)== a && (int)(i*0.1) == b) {
        o = i;
        break;
      }
    }

    System.out.println(o);
  }
} import java.util.Scanner; Scanner java.util public class Main {
  public static void main(final String[] args) {
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    sc.close();

    int o = -1;
    for (int i = 1;i< (int)(100/0.08)+1;i++) {
      if ((int)(i*0.08)== a && (int)(i*0.1) == b) {
        o = i;
        break;
      }
    }

    System.out.println(o);
  }
} Main public static void main(final String[] args) {
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    sc.close();

    int o = -1;
    for (int i = 1;i< (int)(100/0.08)+1;i++) {
      if ((int)(i*0.08)== a && (int)(i*0.1) == b) {
        o = i;
        break;
      }
    }

    System.out.println(o);
  } main {
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    sc.close();

    int o = -1;
    for (int i = 1;i< (int)(100/0.08)+1;i++) {
      if ((int)(i*0.08)== a && (int)(i*0.1) == b) {
        o = i;
        break;
      }
    }

    System.out.println(o);
  } 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 sc.close() sc.close sc int o = -1; o -1 1 for (int i = 1;i< (int)(100/0.08)+1;i++) {
      if ((int)(i*0.08)== a && (int)(i*0.1) == b) {
        o = i;
        break;
      }
    } int i = 1; int i = 1; i 1 i< (int)(100/0.08)+1 i (int)(100/0.08)+1 (int)(100/0.08) 100 0.08 1 i++ i++ i {
      if ((int)(i*0.08)== a && (int)(i*0.1) == b) {
        o = i;
        break;
      }
    } {
      if ((int)(i*0.08)== a && (int)(i*0.1) == b) {
        o = i;
        break;
      }
    } if ((int)(i*0.08)== a && (int)(i*0.1) == b) {
        o = i;
        break;
      } (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 {
        o = i;
        break;
      } o = i o i break; System.out.println(o) System.out.println System.out System System.out o final String[] args args public class Main {
  public static void main(final String[] args) {
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    sc.close();

    int o = -1;
    for (int i = 1;i< (int)(100/0.08)+1;i++) {
      if ((int)(i*0.08)== a && (int)(i*0.1) == b) {
        o = i;
        break;
      }
    }

    System.out.println(o);
  }
} public class Main {
  public static void main(final String[] args) {
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    sc.close();

    int o = -1;
    for (int i = 1;i< (int)(100/0.08)+1;i++) {
      if ((int)(i*0.08)== a && (int)(i*0.1) == b) {
        o = i;
        break;
      }
    }

    System.out.println(o);
  }
} Main
import java.util.Scanner;

public class Main {
  public static void main(final String[] args) {
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    sc.close();

    int o = -1;
    for (int i = 1;i< (int)(100/0.08)+1;i++) {
      if ((int)(i*0.08)== a && (int)(i*0.1) == b) {
        o = i;
        break;
      }
    }

    System.out.println(o);
  }
}