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