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

int main(){
    int a, b;cin >> a >> b;
    if(a > 10 && b > 10){
        cout << -1 << endl;
    }else if(a == 0 || b == 0){
        cout << -1 << endl;
    }else{
        cout << a * b << endl;
    }
}#include <bits/stdc++.h>
using namespace std;

int main(){
    int a, b;cin >> a >> b;
    if(a > 9 && b > 9){
        cout << -1 << endl;
    }else if(a == 0 || b == 0){
        cout << -1 << endl;
    }else{
        cout << a * b << endl;
    }
}