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=2;i<1100;i++) {
				if((int)(i*0.08)==a  && (int)(i*0.1)==b) {
					System.out.println(i);
					return;
				}
			}
			
			
			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