#include <bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/hash_policy.hpp>
using namespace std;
namespace fastIO {
#define BUF_SIZE 100000
#define OUT_SIZE 100000
// fread->read
bool IOerror = 0;
//	inline char nc(){char ch=getchar();if(ch==-1)IOerror=1;return ch;}
inline char nc() {
    static char buf[BUF_SIZE], *p1 = buf + BUF_SIZE, *pend = buf + BUF_SIZE;
    if (p1 == pend) {
        p1 = buf;
        pend = buf + fread(buf, 1, BUF_SIZE, stdin);
        if (pend == p1) {
            IOerror = 1;
            return -1;
        }
    }
    return *p1++;
}
inline bool blank(char ch) {
    return ch == ' ' || ch == '\n' || ch == '\r' || ch == '\t';
}
template <class T>
inline bool read(T &x) {
    bool sign = 0;
    char ch = nc();
    x = 0;
    for (; blank(ch); ch = nc())
        ;
    if (IOerror) return false;
    if (ch == '-') sign = 1, ch = nc();
    for (; ch >= '0' && ch <= '9'; ch = nc()) x = x * 10 + ch - '0';
    if (sign) x = -x;
    return true;
}
inline bool read(double &x) {
    bool sign = 0;
    char ch = nc();
    x = 0;
    for (; blank(ch); ch = nc())
        ;
    if (IOerror) return false;
    if (ch == '-') sign = 1, ch = nc();
    for (; ch >= '0' && ch <= '9'; ch = nc()) x = x * 10 + ch - '0';
    if (ch == '.') {
        double tmp = 1;
        ch = nc();
        for (; ch >= '0' && ch <= '9'; ch = nc())
            tmp /= 10.0, x += tmp * (ch - '0');
    }
    if (sign) x = -x;
    return true;
}
inline bool read(char *s) {
    char ch = nc();
    for (; blank(ch); ch = nc())
        ;
    if (IOerror) return false;
    for (; !blank(ch) && !IOerror; ch = nc()) *s++ = ch;
    *s = 0;
    return true;
}
inline bool read(char &c) {
    for (c = nc(); blank(c); c = nc())
        ;
    if (IOerror) {
        c = -1;
        return false;
    }
    return true;
}
template <class T, class... U>
bool read(T &h, U &... t) {
    return read(h) && read(t...);
}
#undef OUT_SIZE
#undef BUF_SIZE
};  // namespace fastIO
using namespace fastIO;
/************* debug begin *************/
string to_string(string s) { return '"' + s + '"'; }
string to_string(const char *s) { return to_string((string)s); }
string to_string(const bool &b) { return (b ? "true" : "false"); }
template <class T>
string to_string(T x) {
    ostringstream sout;
    sout << x;
    return sout.str();
}
template <class A, class B>
string to_string(pair<A, B> p) {
    return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
template <class A>
string to_string(const vector<A> v) {
    int f = 1;
    string res = "{";
    for (const auto x : v) {
        if (!f) res += ", ";
        f = 0;
        res += to_string(x);
    }
    res += "}";
    return res;
}
void debug_out() { puts(""); }
template <class T, class... U>
void debug_out(const T &h, const U &... t) {
    cout << " " << to_string(h);
    debug_out(t...);
}
#ifdef tokitsukaze
#define debug(...) cout << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__);
#else
#define debug(...) 233;
#endif
/*************  debug end  *************/
#define mem(a, b) memset((a), (b), sizeof(a))
#define MP make_pair
#define pb push_back
#define fi first
#define se second
#define sz(x) (int)x.size()
#define all(x) x.begin(), x.end()
#define sqr(x) (x) * (x)
using namespace __gnu_cxx;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> PII;
typedef pair<ll, ll> PLL;
typedef pair<int, ll> PIL;
typedef pair<ll, int> PLI;
typedef vector<int> VI;
typedef vector<ll> VL;
/************* define end  *************/
void read(int *x, int l, int r) {
    for (int i = l; i <= r; i++) read(x[i]);
}
void read(ll *x, int l, int r) {
    for (int i = l; i <= r; i++) read(x[i]);
}
void read(double *x, int l, int r) {
    for (int i = l; i <= r; i++) read(x[i]);
}
void println(VI x) {
    for (int i = 0; i < sz(x); i++) printf("%d%c", x[i], " \n"[i == sz(x) - 1]);
}
void println(VL x) {
    for (int i = 0; i < sz(x); i++)
        printf("%lld%c", x[i], " \n"[i == sz(x) - 1]);
}
void println(int *x, int l, int r) {
    for (int i = l; i <= r; i++) printf("%d%c", x[i], " \n"[i == r]);
}
void println(ll *x, int l, int r) {
    for (int i = l; i <= r; i++) printf("%lld%c", x[i], " \n"[i == r]);
}
/*************** IO end  ***************/
void go();
int main() {
#ifdef tokitsukaze
    freopen("TEST.txt", "r", stdin);
#endif
    go();
    return 0;
}
const int INF = 0x3f3f3f3f;
const ll LLINF = 0x3f3f3f3f3f3f3f3fLL;
const double PI = acos(-1.0);
const double eps = 1e-6;
const int MAX = 2e5 + 10;
const ll mod = 998244353;
/*********************************  head  *********************************/

using namespace __gnu_pbds;
int n,m;
ll ans;

void go() {
    read(n,m);
    if(n*m>81){
        puts("-1");
    }else{
        printf("%d\n",n*m);
    }
}
