#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, ans;
  cin >> a >> b;
  if (a >= 1 || a <= 9 || b >= 1 || b <= 9) {
    ans = a * b;
  } else {
    ans = -1;
  }
  cout << ans << endl;
  return 0;
}
