#include <bits/stdc++.h>
using namespace std;

int main() {
  int A B,C;
  cin >>A >>B;
  C=A*B;
  if(C/1>9){
    cout << -1;
  }else if(C/9>9){
    cout << -1;
  }else if(C/8>9){
    cout << -1;
  }else if(C/7>9){
    cout << -1;
  }else if(C/6>9){
    cout << -1;
  }else if(C/5>9){
    cout << -1;
  }else if(C/4>9){
    cout << -1;
  }else if(C/3>9){
    cout << -1;
  }else if(C/2>9){
    cout << -1;
  }else if(C/1>9){
    cout << -1;
  }else{
    cout << C;
  }
  
}
