#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b;
  cin >> a >> b;
  if (a % 10 == 0 || b % 10 == 0) {
    cout << "-1";
    return 0;
  }
  if (a >= 10 && b >= 10) {
    cout << "-1";
    return 0;
  }
  cout << a * b;
  return 0;
}
