import java.util.*;
public class Main{
	public static void main(String args[]){
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();
		int ans = -1;
		int max = 101 * 10;
		for(int i = 1;i < max;i++){
			boolean flg = false;
			if((int)(i * 0.08) == A && (int)(i * 0.1) == B){
					flg = true;
					ans = i;
					break;
			}
			if(flg){
				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