#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int a, b;
  cin >> a >> b;
  if (a >= 1 && b >= 1 && a <= 10 && b <= 10)
    cout << a * b << "\n";
  else
    cout << "-1\n";
  return 0;
}
