#include <bits/stdc++.h>
using namespace std;
int main() {
  int A, B;
  cin >> A;
  cin >> B;
  if (1 < A < 20)
    if (1 < B < 20) {
      cout << A * B << endl;
    } else {
      cout << -1 << endl;
    }
  return 0;
}
