import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) throws Exception {

        InputStreamReader is = new InputStreamReader(System.in);
        BufferedReader reader = new BufferedReader(is);

        List<String> mylist = new ArrayList<>();

        // 標準入力からの値を変数strinputに代入
        String strinput = reader.readLine();

        while (strinput != null) {

            mylist.add(strinput);
            strinput = reader.readLine();

        }

        String arr01[] = mylist.get(0).split(" ");

        int a = Integer.valueOf(arr01[0]);
        int b = Integer.valueOf(arr01[1]);

        final double zeiritu8 = 0.08;
        final double zeiritu10 = 0.1;

        for (int i = 1; i <= 1000; i++) {

            double zei8 = i * zeiritu8;
            double zei10 = i * zeiritu10;

            //            System.out.println(i + " " + (int) zei8 + " " + (int) zei10);

            if ((int) zei8 == a && (int) zei10 == b) {
                System.out.println(i);
                return;
            }

        }

        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