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