
import java.util.Scanner;
//
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		//━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
		double a =sc.nextInt();
		double b =sc.nextInt();
		int ans =-1;
		
		for (double d=1;d<=10000;d++) {
			int ai =(int)(0.08*d);
			int bi =(int)(0.1*d);
			if (ai==a&&bi==b) {
				ans = (int)(d);
				break;
			}
		}
		System.out.println(ans);
		
		//━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
		}
		//----------------------------------------
		//メソッド置き場
		
		//----------------------------------------
	}
/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