import java.util.*;

public class Main {
public static void main(String[] args) {
		
		Scanner keyboard = new Scanner(System.in); 
		
		int A  = keyboard.nextInt();
		int B  = keyboard.nextInt();
		
		int ans = -1;
		
		for(int i = 1; i<1500; i++) {
			if(Math.floor(i*0.08) == A && Math.floor(i*0.10) == B) {
				ans = i;
				break;
			} 
		}
				
		System.out.print(ans);
		keyboard.close();	
	}
}
/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