#include <bits/stdc++.h>
using namespace std;
int main() {
  int a, b, cnt = 0;
  bool f = false;
  cin >> a >> b;
  for (int i = 1; i <= 9; i++) {
    for (int j = 1; j <= 9; j++) {
      if (a * b == i * j) {
        f = true;
      }
    }
  }
  if (f)
    cout << a * b << endl;
  else
    cout << "-1" << endl;
  return 0;
}
