import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
        int a = sc.nextInt();
        int b = sc.nextInt();
		for(int i = 1;i <= 1000;i++){
          if(a == (int)(i * 0.08) && b == (int)(i * 0.1)){
            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);
		// 整数の入力
        int a = sc.nextInt();
        int b = sc.nextInt();
		for(int i = 1;i <= 1000;i++){
          if(a == (int)(i * 0.08) && b == (int)(i * 0.1)){
            System.out.println(i);
            return;
          }
        }
      System.out.println(-1);
    }
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
        int a = sc.nextInt();
        int b = sc.nextInt();
		for(int i = 1;i <= 1000;i++){
          if(a == (int)(i * 0.08) && b == (int)(i * 0.1)){
            System.out.println(i);
            return;
          }
        }
      System.out.println(-1);
    } main {
		Scanner sc = new Scanner(System.in);
		// 整数の入力
        int a = sc.nextInt();
        int b = sc.nextInt();
		for(int i = 1;i <= 1000;i++){
          if(a == (int)(i * 0.08) && b == (int)(i * 0.1)){
            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 = 1;i <= 1000;i++){
          if(a == (int)(i * 0.08) && b == (int)(i * 0.1)){
            System.out.println(i);
            return;
          }
        } int i = 1; int i = 1; i 1 i <= 1000 i 1000 i++ i++ i {
          if(a == (int)(i * 0.08) && b == (int)(i * 0.1)){
            System.out.println(i);
            return;
          }
        } {
          if(a == (int)(i * 0.08) && b == (int)(i * 0.1)){
            System.out.println(i);
            return;
          }
        } if(a == (int)(i * 0.08) && b == (int)(i * 0.1)){
            System.out.println(i);
            return;
          } a == (int)(i * 0.08) && b == (int)(i * 0.1) a == (int)(i * 0.08) a (int)(i * 0.08) i 0.08 b == (int)(i * 0.1) b (int)(i * 0.1) i 0.1 {
            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);
		// 整数の入力
        int a = sc.nextInt();
        int b = sc.nextInt();
		for(int i = 1;i <= 1000;i++){
          if(a == (int)(i * 0.08) && b == (int)(i * 0.1)){
            System.out.println(i);
            return;
          }
        }
      System.out.println(-1);
    }
} public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
        int a = sc.nextInt();
        int b = sc.nextInt();
		for(int i = 1;i <= 1000;i++){
          if(a == (int)(i * 0.08) && b == (int)(i * 0.1)){
            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);
		// 整数の入力
        int a = sc.nextInt();
        int b = sc.nextInt();
		for(int i = 1;i <= 1000;i++){
          if(a == (int)(i * 0.08) && b == (int)(i * 0.1)){
            System.out.println(i);
            return;
          }
        }
      System.out.println(-1);
    }
}