#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b;
  cin >> a >> b;
  if (a > 9 || b >> 9 || a <= 0 || b <= 0) {
    cout << -1 << "\n";
    return 0;
  }
  cout << a * b << "\n";
  return 0;
}
