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();
		int rangeStart = B * 10;
		int ans = -1;

		for(int i = rangeStart; i < rangeStart + 10; i++) {
			if(A == (int)Math.floor(i*0.08)) {
				ans = i;
				break;
			}
		}
		System.out.println(ans);


	}
}

 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();
		int rangeStart = B * 10;
		int ans = -1;

		for(int i = rangeStart; i < rangeStart + 10; i++) {
			if(A == (int)Math.floor(i*0.08)) {
				ans = i;
				break;
			}
		}
		System.out.println(ans);


	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();
		int rangeStart = B * 10;
		int ans = -1;

		for(int i = rangeStart; i < rangeStart + 10; i++) {
			if(A == (int)Math.floor(i*0.08)) {
				ans = i;
				break;
			}
		}
		System.out.println(ans);


	} main {
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();
		int rangeStart = B * 10;
		int ans = -1;

		for(int i = rangeStart; i < rangeStart + 10; i++) {
			if(A == (int)Math.floor(i*0.08)) {
				ans = i;
				break;
			}
		}
		System.out.println(ans);


	} 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 int rangeStart = B * 10; rangeStart B * 10 B 10 int ans = -1; ans -1 1 for(int i = rangeStart; i < rangeStart + 10; i++) {
			if(A == (int)Math.floor(i*0.08)) {
				ans = i;
				break;
			}
		} int i = rangeStart; int i = rangeStart; i rangeStart i < rangeStart + 10 i rangeStart + 10 rangeStart 10 i++ i++ i {
			if(A == (int)Math.floor(i*0.08)) {
				ans = i;
				break;
			}
		} {
			if(A == (int)Math.floor(i*0.08)) {
				ans = i;
				break;
			}
		} if(A == (int)Math.floor(i*0.08)) {
				ans = i;
				break;
			} A == (int)Math.floor(i*0.08) A (int)Math.floor(i*0.08) Math.floor Math i*0.08 i 0.08 {
				ans = i;
				break;
			} ans = i ans i break; System.out.println(ans) System.out.println System.out System System.out ans 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();
		int rangeStart = B * 10;
		int ans = -1;

		for(int i = rangeStart; i < rangeStart + 10; i++) {
			if(A == (int)Math.floor(i*0.08)) {
				ans = i;
				break;
			}
		}
		System.out.println(ans);


	}
}


