import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();
		double res = 0;
		while (res<=20000) {
			if(res * 0.08 >= A && res * 0.08 < A + 1 && res * 0.1 >= B && res * 0.1 < B + 1) {
				break;
			}
			res++;
		}
		if((int)res!=20001) {
			System.out.println((int)res);
		}else {
			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