import java.io.*;
import java.util.*;

public class Main {

    public static void main(final String[] args) throws Exception {
        final Scanner sc = new Scanner(System.in);
        long A;
        A = sc.nextLong();
        long B;
        B = sc.nextLong();

        final Solver solver = new Solver();
        solver.solve(A, B);
    }
}

class Solver {
    public void solve(long A, long B) {
        if (A <= 0 || A >= 10 || B <= 0 || B >= 10) {
            System.out.println(-1);
            return;
        }
        System.out.println(A * B);
    }
}


/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