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 = 1; i < 1100;i++ ) {
			int eight = i * 8 /100;
			int ten = i * 10 /100;
			if(eight ==a && ten == 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 = 1; i < 1100;i++ ) {
			int eight = i * 8 /100;
			int ten = i * 10 /100;
			if(eight ==a && ten == 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 = 1; i < 1100;i++ ) {
			int eight = i * 8 /100;
			int ten = i * 10 /100;
			if(eight ==a && ten == 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 = 1; i < 1100;i++ ) {
			int eight = i * 8 /100;
			int ten = i * 10 /100;
			if(eight ==a && ten == 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 = 1; i < 1100;i++ ) {
			int eight = i * 8 /100;
			int ten = i * 10 /100;
			if(eight ==a && ten == b) {
				System.out.println(i);
				return;
			}
		} int i = 1; int i = 1; i 1 i < 1100 i 1100 i++ i++ i {
			int eight = i * 8 /100;
			int ten = i * 10 /100;
			if(eight ==a && ten == b) {
				System.out.println(i);
				return;
			}
		} {
			int eight = i * 8 /100;
			int ten = i * 10 /100;
			if(eight ==a && ten == b) {
				System.out.println(i);
				return;
			}
		} int eight = i * 8 /100; eight i * 8 /100 i * 8 i 8 100 int ten = i * 10 /100; ten i * 10 /100 i * 10 i 10 100 if(eight ==a && ten == b) {
				System.out.println(i);
				return;
			} eight ==a && ten == b eight ==a eight a ten == b ten 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 = 1; i < 1100;i++ ) {
			int eight = i * 8 /100;
			int ten = i * 10 /100;
			if(eight ==a && ten == 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 = 1; i < 1100;i++ ) {
			int eight = i * 8 /100;
			int ten = i * 10 /100;
			if(eight ==a && ten == 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 = 1; i < 1100;i++ ) {
			int eight = i * 8 /100;
			int ten = i * 10 /100;
			if(eight ==a && ten == b) {
				System.out.println(i);
				return;
			}
		}
		System.out.println(-1);
	}
}