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