import java.util.Scanner;

public class Main {

	public static void main(String args[]){

		Scanner scan = new Scanner(System.in);
		int a = scan.nextInt();
		int b = scan.nextInt();
		int min = -1;
		
		for(int i = 1; i<=10000;i++){
			if((int)(i*0.08) == a && (int)(i*0.1) == b){
				min = i;
				break;
			}
		}
		
		System.out.println(min);
		
	}
}
/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