import java.util.Scanner;
public class Main {

	public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);

		int a = stdIn.nextInt();
		int b = stdIn.nextInt();
		
		double c = a/0.08;
		double cc = (a+1)/0.08;

		
		double d = b/0.1;
		double dd = (b+1)/0.1;
		
		boolean flag = true;
		
		Outer:for(int i =(int) Math.ceil(c); i < cc; i++) {
			
			for(int j = (int)Math.ceil(d); j < dd; j++) {
				if(i == j) {
					System.out.println(i);
					flag = false;
					break Outer;
				}
				
			}
			
		}
		
		if(flag) {
			
		System.out.println("-1");
		}
		
		

	}

}

/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz