#include <bits/stdc++.h>
using namespace std;
int N;
int len[1000000 + 10];
int result = 0;
int main() {
  int a, b;
  cin >> a >> b;
  if (a > 9 || b > 9)
    cout << -1 << endl;
  else {
    cout << a * b << endl;
  }
}
