//package atcoder;
 
import java.util.*;
import java.math.BigDecimal;
 
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<=1000;i++) {
			if((int)(i*0.08)==A&&(int)(i*0.1)==B) {
				System.out.println(i);
				System.exit(0);
			}
		}
		System.out.println(-1);
		
		}
} import java.util.*; util.* java import java.math.BigDecimal; BigDecimal java.math 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<=1000;i++) {
			if((int)(i*0.08)==A&&(int)(i*0.1)==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<=1000;i++) {
			if((int)(i*0.08)==A&&(int)(i*0.1)==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<=1000;i++) {
			if((int)(i*0.08)==A&&(int)(i*0.1)==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<=1000;i++) {
			if((int)(i*0.08)==A&&(int)(i*0.1)==B) {
				System.out.println(i);
				System.exit(0);
			}
		} int i=0; int i=0; i 0 i<=1000 i 1000 i++ i++ i {
			if((int)(i*0.08)==A&&(int)(i*0.1)==B) {
				System.out.println(i);
				System.exit(0);
			}
		} {
			if((int)(i*0.08)==A&&(int)(i*0.1)==B) {
				System.out.println(i);
				System.exit(0);
			}
		} if((int)(i*0.08)==A&&(int)(i*0.1)==B) {
				System.out.println(i);
				System.exit(0);
			} (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);
				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 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<=1000;i++) {
			if((int)(i*0.08)==A&&(int)(i*0.1)==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<=1000;i++) {
			if((int)(i*0.08)==A&&(int)(i*0.1)==B) {
				System.out.println(i);
				System.exit(0);
			}
		}
		System.out.println(-1);
		
		}
} Main
//package atcoder;
 
import java.util.*;
import java.math.BigDecimal;
 
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<=1000;i++) {
			if((int)(i*0.08)==A&&(int)(i*0.1)==B) {
				System.out.println(i);
				System.exit(0);
			}
		}
		System.out.println(-1);
		
		}
}
