#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pp = pair<ll, ll>;
const ll amari = 1e9 + 7;
const char BC = 'A' - 'a';
int main() {
  int a, b;
  cin >> a >> b;
  if (a > 10 || b > 10)
    cout << -1 << endl;
  else
    cout << a * b << endl;
  return 0;
}
