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);


	}
}



/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