import java.util.Scanner;

public class Main {
	public static void main(String args[]) {
		Scanner scan = new Scanner(System.in);
		int eight = scan.nextInt(), ten = scan.nextInt();
		
		for(int i = ten * 10; i < ten * 10 + 10; i++) {
			if((int)(i * 0.08) == eight) {
				System.out.println(i);
				System.exit(0);
				
			}
			
		}
		
		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