import java.util.*;

public class Main {
	public static void main(final String[] args) {

        Scanner sc = new Scanner(System.in);

        // 8%時の税抜価格
        double a = sc.nextDouble() / 0.08;

        // 10%時の税抜価格
        double b = sc.nextDouble() / 0.1;

        // 税抜き価格 最小値判別
        if ((b <= a) && (a <= b + 9)) {

            System.out.println((int)Math.ceil(a));
        } else if ((a <= b) && (b <= a + 12)) {

            System.out.println((int)Math.ceil(b));
        } else {

            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