#!/usr/bin/env python

from collections import deque, defaultdict
import itertools as ite
import sys
import math
import decimal

sys.setrecursionlimit(1000000)

INF = 10 ** 18
MOD = 10 ** 9 + 7

A, B = map(int, raw_input().split())
if A > 9 or B > 9:
    print -1
else:
    print A * B