#include <bits/stdc++.h>
using namespace std;
using ll = long long;
void self_max(int &a, int b) { a = max(a, b); }
void self_min(ll &a, ll b) { a = min(a, b); }
int main() {
  int a, b;
  cin >> a >> b;
  if (a >= 1 && a <= 9) {
    cout << -1 << endl;
  } else if (b >= 1 && b <= 9) {
    cout << -1 << endl;
  } else {
    cout << a * b << endl;
  }
  return 0;
}
