//package atcoder;
 
import java.util.*;
import java.math.BigDecimal;
 
public class Main{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();
		for(int i=0;i<=1000;i++) {
			if((int)(i*0.08)==A&&(int)(i*0.1)==B) {
				System.out.println(i);
				System.exit(0);
			}
		}
		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