#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main(void) {
  int a, b;
  cin >> a >> b;
  if (1 <= a && a <= 20 && 1 <= b && b <= 20)
    cout << a * b << '\n';
  else
    cout << "-1" << '\n';
  return 0;
}
