import java.util.*;

class Main {
	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		int a = in.nextInt();
		int b = in.nextInt();
		int result = -1;

		for(int i=1; i<=1250; i++){
			if(Math.floor(i*0.08)==a && Math.floor(i*0.1)==b){
				result = i;
				break;
			}
		}
		System.out.println(result);

	}
}

/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