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