#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9 + 10;
const long long int LINF = 1LL * INF * INF;
const int MAXN = 2e5 + 10;
const int MAXM = 5e3 + 10;
priority_queue<int> pq;
vector<vector<int> > graph;
queue<int> que;
int cal[110];
int main() {
  int n, m, k, a, b, x, y, q;
  int sum = 0;
  int cnt = 0;
  int mx = 0;
  int mn = INF;
  int cur = 0, idx = -1;
  int tc;
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cin >> a >> b;
  for (int i = 1; i <= 9; i++)
    for (int j = 1; j <= 9; j++) cal[i * j]++;
  if (cal[a * b])
    cout << a * b << "\n";
  else
    cout << "-1\n";
  return 0;
}
