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