#include <bits/stdc++.h>
using namespace std;
const int mod = 1e9 + 7;
signed main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int a, b;
  cin >> a >> b;
  int ans = a * b;
  if (0 < ans && ans < 100) {
    cout << ans;
  } else {
    cout << -1;
  }
}
