import java.util.Scanner;

	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=2;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.Scanner; Scanner 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=2;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=2;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=2;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=2;i<1100;i++) {
				if((int)(i*0.08)==a  && (int)(i*0.1)==b) {
					System.out.println(i);
					return;
				}
			} int i=2; int i=2; i 2 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=2;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=2;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.Scanner;

	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=2;i<1100;i++) {
				if((int)(i*0.08)==a  && (int)(i*0.1)==b) {
					System.out.println(i);
					return;
				}
			}
			
			
			System.out.println(-1);
			
		}

	}