import java.util.*;

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<1001; i++) {
			if(i*8/100==A && i*10/100==B){
				System.out.println(i);
				return;
			}
		}
		System.out.println(-1);
	}
} import java.util.*; util.* java 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<1001; i++) {
			if(i*8/100==A && i*10/100==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=0; i<1001; i++) {
			if(i*8/100==A && i*10/100==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=0; i<1001; i++) {
			if(i*8/100==A && i*10/100==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=0; i<1001; i++) {
			if(i*8/100==A && i*10/100==B){
				System.out.println(i);
				return;
			}
		} int i=0; int i=0; i 0 i<1001 i 1001 i++ i++ i {
			if(i*8/100==A && i*10/100==B){
				System.out.println(i);
				return;
			}
		} {
			if(i*8/100==A && i*10/100==B){
				System.out.println(i);
				return;
			}
		} if(i*8/100==A && i*10/100==B){
				System.out.println(i);
				return;
			} i*8/100==A && i*10/100==B i*8/100==A i*8/100 i*8 i 8 100 A i*10/100==B i*10/100 i*10 i 10 100 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
import java.util.*;

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<1001; i++) {
			if(i*8/100==A && i*10/100==B){
				System.out.println(i);
				return;
			}
		}
		System.out.println(-1);
	}
}